Added player implementation.

* Entity.cs, Button.cs, State.cs, MenuState.cs, DifficultyState.cs:
  Modified to use float instead of double for delta time.

* Player.cs:
  Initial implementation.

* Game.cs:
  Cast DeltaTime to float from double.

* Input.cs:
  Cleanup.

* GameState.cs:
  Added EndGame().
  Added creation / reset code for player.
  Modified to use float instead of double for delta time.

* Extensions.cs:
  Added MathUtil.Clamp.
  Added MathUtil.Lerp.