Keyboard capture and game code override

• Keyboard will only capture from game canvas when it has focus. This makes it less frustrating to interact with the browser while the game is on the page and it promotes the same behaviour as users are used to from flash games.
• The punchdrunk constructor now takes a game_code option which accepts some moonshine-style jsonized byte-code to override the built-in lua boot code and run directly. It's nice for small, single file scripts.