Added new features for v1.2 of the game

* Each game now starts off with 3 lives. You lose a life each time the
ball misses the paddle and hits the bottom of the screen.
* 2 new levels have been added, giving 5 levels in total.
* A couple of bugs have been fixed - to make sure the "Level 1" text is
always displayed at the top of a new game, and to make sure the smiley
ball doesn't move off the screen.