Flxpong

This pull request adds FlxPong to the flixel-demos repository per my suggestion in issue #45.

The game itself is very simple; more or less a roguelike Pong where you face against a series of successively faster and larger enemies. However, the menu allows simple access to test the entire FlxGameJolt class, with a button and return information for each function.

Furthermore, the game itself awards trophies, sets high scores, and has a login screen should the game not be able to auto-authenticate user data. Future revisions to this demo may incorporate even more of FlxGameJolt's features, and is meant to demonstrate their use for developers.

This has been tested in Flash and Windows. Updated Flash and Windows versions will be available on GameJolt shortly.