r3.4

1. Stages now use a timer instead of lives
2. The Life counter no longer exists and instead is replaced by Timer.
When the time runs out, you lose.
3. Score is based on brick multiplier times time remaining on clock.
Thus more high-value bricks give more points if destroyed earlier.
Similarly, if you beat a stage with time left, that adds to the time on
the next stage, thus increasing scores.
4. Moved main() code into an Arkanoid class to make it easier to break
parts of the execution into smaller functions.