Use cancelAnimationFrame in Ticker.pause() if animation frames are enabled

This is implemented by capturing the callback reference returned by
requestAnimationFrame which can be cancelled in a similar manner to
clearing a timeout.

Prior to this, Ticker.pause() would clear sjs.animationFrame and then
a null reference error would be triggered when resuming.
Merge pull request #31 from danielwhite/pause-for-animation-frames

Use cancelAnimationFrame in Ticker.pause() if animation frames are enabled