Port Breakout to current Web Animations API

Breakout now supplies durations in milliseconds instead of seconds.

Animations explicitly fill forwards instead of relying on the old default (the default is now fill none).

We use player finish events instead of timed item end events.

We now protect against 'Play Again' being clicked more than once.
Merge pull request #30 from ewilligers/breakout-using-fill-forwards

Port Breakout to current Web Animations API