By taking Ghost, you can gank a helluvalot faster than you normally can. Ghost also helps when chasing down low level targets or an attempt to save one of your laners. Smite is just a good choice when jungling, for you can steal buffs easily or clear camps faster.

Hunter's Machete is a necessary early game item, for most junglers, a 10% increase in damage helps a lot. Health Potions aren't very needed when you intend on lurking in the jungle, but when I play jungle Warwick, I play offensive and sometimes you'll lose a considerable amount of health.
Cloth Armor will prevent you from losing much health from camps, increasing your sustain and ability to stay in the jungle, while Boots of Speed will enable you to move from lane to lane or camp to camp a lot faster.
Taking two daggers will improve your ability to clear jungle camps and gank, while Amplifying Tome will increase the damage of your Q as well as the heal from it.
Upgrading to Ninja Tabi will improve your ability to gank and clear jungle camps, as well as reduce the damage taken when you gank.
Taking Warden's Mail will further improve your survivability and the passive helps you get away or take epic monsters, such as the Ancient Golem, faster without losing much health. Kindlegem will give you more health and cooldown reduction, as well as build into one of the most important items to take as Warwick. Brutalizer will speed up your kills in general.
Finally, Black Cleaver will make your damage escalate, while Spirit Visage and Randuin's Omen will work to keep you alive in those fights where you're at a disadvantage. Ravenous Hydra will dominate team fights, as well as keep you alive.

If you followed the Mastery and Rune suggestions, early game clears should be no sweat. By taking your W first, you can clear camps a lot faster than you normally would. Next, take your Q, to increase your damage output and sustain. Have a point in your W once again, then gank. Take another point in your Q. My suggestion, take your Ult at level 6,11,and 16, and max your Q by level 14, and your E by level 17, and W by level 18. Your dependency on your W will decrease as you buy items.