Implemented projectile loading and removal when lifetime expires
Refactored game logic to GameController from StarBattle

I like this flow better as it leads to a less convoluted StarBattle class, which can handle overarching game control issues such as loading textures, managing screens, etc
On the other hand, GameController handles the finer details of the currently running game
Added basic (brute force) collision detection

Projectiles collide and do damage to ships!