Jetpack Joyride Clone Added

Finishing the "Game 2" section, I've added "Jetpack" (I guess it's not a particularly joyful ride...). So far, I feel like this game's the one that has the most room for further refinement. It feels a bit wonky to control, and the obstacles aren't particularly engaging, but nevertheless I think it's still in a sufficient "done" state, at least for now. Like Breakout, I might come back to it later and try to improve it. Now, however, it's on to "Game 3".

In addition, I've updated how high scores work; they now persist when switching games. I've also added a button on the main menu to view your high scores.