Anithing you think that can be different to make this a better guide or any doubt jusk ask me and I will be glad to improve or help you.
That being said, lets get started.

You might be thinking "WTF is this build!". But think about it... Your team already has a AD carry, AP carry, AP tank (like Amumu or Maokai) and support. Your team would be glad to have an AD offtank jungler with a freaking awesome slow. No one can escape from you. Try this build before saying it won't work.

Pros:
[*] Easy to play;
[*] Easy jungle (thanks to Consume);
[*] Runs fast as hell;
[*] Nice atk speed and damage;
[*] Hard to kill;
[*] Freaking awesome slow combo.
Cons:
[*] Not as much damage with skills as AP (But you will stil slows and do some dmg with them);
[*] May run out of mana with the second biuld (nothing a blue buff or Soraka can't solve)

Greater Seal of Armorx9 for survival in the jungle;

Greater Glyph of Attack Speedx9 for faster jungling;

Greater Mark of Attack Damagex9 for faster jungling.
You will do it way too fast and you will do a nice damage at early ganks.

Greater Quintessence of Movement Speedx3 are for almost 500 movement speed at lvl 18 when using Blood Boil with the first build and 500 movement speed WITHOUT Blood Boil with the second, and great mobility at lower lvls. That helps your ganks, escaping ganks and couter-jungling.

The best:
Smite - Any doubt about it? You are a jungler...
Flash - Well, I think this is one of the best Summoner Spells. It gives you mobility to chase, escape and even help if you are cought counter-jungling.

You can also chose:
Teleport (if your team has someone like Teemo so you can teleport to his shroms);

Heal (if you are not lvl 30 nor have all runes and are not capable of jungling without healing yourself);

Ghost (but you already have a speed up bonus from Blood Boil);

Exhaust ([but you already have 2 slows from Ice Blast and Frozen Mallet);

Cleanse (good if the opponent team has a lot of CC, but how can you know it before the game starts?);

Ignite (are you planing on letting anyone escape? I don't think you need it) if you wish.

The rest aren't good at all to you.

Now you are doing an AD Nunu & Willump. So chose AD masteries. Don't forget that you are also a jungler, so you need more exp, granted by Awareness .
Another option is focusing deffence. You are also an off-tank. So you can get more deffence and yet deploy a nice damage. And again, if you are doing jungle, don't forget Awareness .

Every AD damage dealer normaly buy a pair of boots, one defencive, 2 damage and 2 atk speed itens. Thanks to your Blood Boil you don't need that much atk speed so you are doing 3 damage, 1 defencive and 1 defencive item that gives your some damage ( Frozen Heart or Atma's Impaler)

To start I aways buy a Ruby Crystal (with both builds). It gives me a great amount of health, good for jungling, since I have atk speed and damage from my runes.

Later on, I buy a Boots and Tear of the Goddess. Before doing Manamune I buy a Phage and Mercury's Treads so I can go gank and slows (even more) my poor opponents. The others itens depends on how the game is going. But you can do whatever item you wish. You will get OP anyways.

For the second build I recomend going for Boots, Zeal, Phage. As Zeal give you some movement speed, you can finish your boots after Phage.

Lets talk about your mana pool. With Manamune and Frozen Heart you won't have any problem with mana as you have 2300 mana. So what you did was sacrificing some atk speed to get mana to spam Blood Boil and Ice Blast nonstop. Is it worth it? Depends on your playstyle. If you realy want atk speed to stack with the atk speed bonus from Blood Boil chose the second build. Without mana and mana regen items you MAY run out of mana. But every 7 atacks you have a free spell. So it is a good idea getting atk speed so you can use free spells. That is the bad thing about not having mana regen and large mana pool. So what is the good thing about it? The aswer: more atk speed = more damage, Phanton Dancer + Atma's Impaler + Infinity Edge = even more damage AND BIG CRITS... Chose wisely.

Consume lvl 2 helps even more your jungling and counter-jungling. But since you are not going to spend all your time killing neutral minions, get it just to lvl 2.
Blood Boil is what makes and AD Nunu & Willump scary. So you have to get it before Ice Blast. Blood Boil also makes your jungling easyer.
Altering Blood Boil and Ice Blast is perfect. Your movement and atack speed and slows get to a higher % at the same time, what helps your teammates as well as yourself.
Of course you must not forget you ultimate Absolute Zero. It is true you are not doing as much damage as if you were AP but you will stil slows and be capable of doing more than 1000 damage with it.

As a jungler you have to know more than one route. But for those who don't even lnow how to start it, here it comes:

I usually start at Blue buff, killing the big golem first by hitting he once, using Smite, hit a couple more times, use Consume so you can use it efectly restoring my health lost and just auto atack.

Next, the wolves. Always killing the big one first.

Than wraths, get the Red buf and two golems.

Gratz, you just got lvl 4 and spend the first skill point at Ice Blast. Now go use it. GANK!

Remember: only gank if you have enought health to do it...

At this time you are lvl 4 to 5 and are free to do whatever you want (gank more, jungle til lvl 6, stay in lane), but I recomend you go back to base to buy some items, gank if there is any opportunity and get lvl 6 to unlock your champion ultimate ability.