Added scoreboard.

* ScoreboardState.cs:
  Initial version.

* Game.cs:
  Added ScoreboardState.

* DifficultyState.cs:
  Set the default difficulty index to 1 (normal).

* EndGameState.cs:
  Added ScoreboardState functionality and added buttons to view the top scores and go back to the menu.

* GameState.cs:
  Converted Score to int from uint for compatibility with other classes.

* MenuState.cs:
  Now draws the game in the background.

* Extensions.cs:
  Added SFMLUtil.Size() for SFML text objects.