Added basic scoring system.

* Block.cs:
  Initial version.

* MenuState.cs, EndGameState.cs:
  Removed redundant checking.

* BlockSpawner.cs:
  Modified to use Block class instead of RectangleShape.

* Player.cs:
  Cleaned up and refactored a bit.

* GameState.cs:
  Score resets on new game.
  Removed redundant checking.