Added documentation. Added collision handler for blocks and the player.

* BlockSpawner.cs:
  Documentation.

* GameScene.cs:
  Documentation.
  Added separate Player and BlockSpawner properties.
  Added regions.
Added blank end game scene. Start of UI.

* Button.cs, EndGameScene.cs:
  Initial version.

* Game.cs:
  Renamed OnEndFrame event to EndFrame.

* Input.cs:
  Added MouseMoved event.

* Parameters.cs:
  Made _all readonly.

* Extensions.cs:
  Added Size() and ToVector2f() methods.