added gamestate abstract class, and playscreen implementation
reference to game objects now comes from play screen
screens are now responsible for playing their own music
moved everything for gameplay into the playScreen object
updated todo
changed gamestate base and game state stack
changed all the gamestates and added a new pause state
Game class now uses the state stack
removed static reference to game window
updated todo
rearranged project groups
particle emitter now takes a position in the constructor, and allows for movement
matches new emitter signature