Created abstract class Screen and class Game.

somewhat modling off of the Frogger structure I created an abstract
class Screen. Also create the Game class and give it a setScreen method
that will easily allow swiching from screen in the menu (e.g. credits,
tutoiral) and starting the actual game. (hoping that method will help
when we implement level selection, but im not sure. either way its a
good way to manage the start screens from the game screens).