This Kassadin build focuses on his ability to move quickly across the map to capture towers, especially during the early phase of the game, when the base damage from his nukes are still strong so he can skip getting ap. By getting the cheap tower stealing items early in the game, he can get the 100 gold bonus from captures and neutralizes, which helps build into his late game.

The mastery tree of choice is Utility (+Offense). Key masteries are:
9% CDR
1 gold/10s*
3% move speed bonus
makes the speed boost from shrine last longer.

*The gold/10 is just for the lols, feel free to change it to anything else.

Exhaust is a must have for Kassadin. It can help secure a kill to make captures more likely.
Ghost, which enables you to move even faster!

The key runes are the cdr/level glyphs and quints. With masteries and Deathfire Grasp, you end up with 39% cdr at lv 18.

The early Nether blade at lv4 is good for charging up Force Pulse and Tear of the Goddess.

Force Pulse is maxed first because you don't start with ap in this build, so the aoe slow and damage is better. Also, Force Pulse + Riftwalk can clear creep waves when needed (albeit a little slow).

The ultimate spell for any Tower Thief! Remember to watch the mana when using this spell and try not to get over 3 stacks, especially when rushing to battles; you'll need it for chasing or escaping.

Tower Thief Items

Solves mana problems. Get it first and charge it up.
A must for tower capturer. It speeds up your capture rate so most champions can't stop you in time if they are 1 tower away.
* An alternative. This gives ~50 more speed, quite noticeable.
Get it after Priscilla's. In Dominion many people rush for damage, so the flat penetration really helps.
Catalyst the Protector Survivability.

Late Game Items

After Catalyst, there are only 2 item slots left. Time to get some AP.
Recommended items:

Gives you another nuke and 15% cdr.
The much needed AP. Get Odyn's Veil first though if you kept dying to magic damage. See next line.
or . Upgrade Catalyst into once of these. If you kept dying to magic damage, get Odyn's veil, which also gives you another nuke!
Upgrade Tear of Goddess.

Alternatives

Instead of DFG, you can get lich bane. Comes after Rabadon because you didn't have AP so lich bane makes no sense. This items gives move speed too! Good for those who like to tough it out instead of hit and run (although this item makes you run faster).

Situationals

After finishing Rabaddon's, if the enemy team stacked magic resistance, sell Priscilla's for magic penetration.
If you REALLY want to sell Priscilla's (why would you do that!?) and the enemies are not stacking resistance, get Abyssal Scepter instead of Void Staff.

Do not dedicate yourself to stealing towers. Ain't the title misleading : p

Early to Mid Game

At the spawn, ask your allies to cast spells to charge up your Force Pulse. During the first team fight at the Windmill, try to use Force Pulse as often as possible. There are many champions casting spells, so it could recharge many times.

Usually teams start with the 4 top 1 bot strategy. Kassadin should NOT be the solo. Silence, aoe slow, and exhaust are too good to be missing from the battle at Windmill.

Tower Stealing

Fight with your team untill Priscilla's blessing. Now watch out for open towers and attempt to neutralize if possible. Neutralizing and capturing each gives 100 bonus gold, so this really helps build into your late game.

If you manage to capture the other mid tower, stay around and defend it, especially after you got catalyst. You can safely delay 1 person from capturing, so delay as long as possible. Watch out for enemy reinforcement. When in doubt, run!

A good time to steal tower is when your team tries to capture a tower, but there are too many defending champions (stalemate). Time to abandon your allies and steal a tower! Sometimes you can even get back in time for the fight.

Late Game

During late game (when enemy nexus HP is less than 100), captures and champion kills no longer lowers nexus hp. It's time to finish off the other team.

Kassadin should use hit and run tactic. Start with riftwalk > dfg > force pulse > null sphere, and if kill seems unlikely, rift away. Try to do Force before Null because it gives another charge to Force (Force Pulse casting is instant anyway). During Null Sphere cooldown, charge up your Force pulse to 4 charges with Nether blade and riftwalk if required, then do rift > null > forcepulse for the kill!

It is better to just hold 3 towers instead of trying to capture more. Patrol the middle of the map to provide intel on enemy movements (and maybe get some kills) and support tower under attack. You can also fake tower steals by moving through areas with shared vision with halfhearted capture attempts, which might cause enemy to split up.

Summary

Capture towers

Spam Nether blade, it charges up Tear and Force Pulse and only takes 25 mana.

Use Recall often. It only takes 4.5s in Dominion and you can move across the map fairly quickly so it's not a problem to go back often.

Try not to let riftwalk get over 3 stacks when rushing toward battles, it'll take too much mana when you need it for chasing or escaping

Escape by blinking over walls, into bushes, onto health and speed shrines.
Use Priscilla's blessing's speed boost.

Use summoner's spells often, you got utility mastery.

In the beginning I tried this item build for 0 ap Kassadin which took less than 9k gold to finish:
catalyst the protector
http://www.mobafire.com/league-of-legends/item-purchase-planner#&item=58-1-123-86-110-42-39-124

With this build, he is very hard to kill, but lacks damage late game. In the beginning, he still kills fine because the base damage from his spells are sufficient plus he chases well, but the damage totally falls off the cliff late game, making him virtually useless. The enemy team can just hold 3 towers and he can't steal when there is 1 champion defending because the lack of damage. In addition, captures no longer damages enemy nexus. This is why he still needs AP and should not be built tanky.
You can usually get close games though, because if you manage to neutralize and capture a tower, it's -5 nexus hp before late game!

Tower Thief Kassadin is most noticeable in the early game, during which he gives up damage for Priscilla's and Catalyst. His mobility enables him to distract the enemy team and get himself bonus gold! Throughout the game, try to capture as many towers as possible with Priscilla's. Please give it a try and leave comments or suggestions.