Set up Sound Manager and successfully played sounds and music

-Added:
  -SoundManager.cs
  -test sounds and music
Finished Menu System

-Added Stack of MenuScreen objects to Main class
-Added virtual PickOption method in MenuScreen class
-Added ResetInput method to MenuScreen class
-Finished screen transitions
-Added global GameState enum, and made it possible for the player to be in different game states
Merge remote-tracking branch 'origin/MenuScreenTransitions' into SoundSystem
Merge branch 'SoundSystem'