Added SimpleBullets and introduced the SpriteList class

An abstract Projectiles class has been added, with a concrete subclass SimpleBullet. The SimpleTank has been given a method to fire the SimpleBullet (this will be expanded to fire all types of Projectiles) and a ShootOrder has been written to make it do that.

I added a test case to the GameTest for firing these projectiles, and they seem to work, though it's hard to tell; a better test case needs writing.

Do to complications that arose when Sprites were created and destroyed during the main game loop (something that had not been tested until now), I wrote a SpriteList class to wrap some operations that had to be performed on the sprites ArrayLists.