Added a small tutorial to explain the mechanisms of Zombies Against Us.

Example Screen :

We would really appreciate feedbacks about the tutorial, to improve it if it needs to be. We created some documentation/information into the game. It's called specifics and you can find (for now) infos about enemies, turrets and icons.

For example, here is a part of the enemies specifics :

Finally, we added a built-in update checker, so people using the browser download won't miss the latest version.