Added menu and difficulty state.

* Difficulty.cs, GameState.cs, GameDifficulty.cs,
   DifficultyState.cs, DifficultySettings.cs:
  Initial version.

* Game.cs:
  Added Game.Size.
  More documentation.
  Extra error handling.

* Program.cs:
  Added Program.Name and Program.DefaultState.

* MenuState.cs:
  Added state title text.
  Added menu buttons.

* Button.cs:
  Added Center().
  Removed useless Font variable.

* Extensions.cs:
  Added Round() and ToFloat().