Basic game working, some cleanup and bugfix necessary.

Missing game over, score, etc., but the scaffold is there. A lot of work
late at night so code probably needs cleanup.

Haven't had a second to fix yet, but simple extant bug:

    4  4  8  4  --LEFT ARROW-->  16  4  .  .

should instead be:

    4  4  8  4  --LEFT ARROW-->   8  8  4  .