Add requestAnimationFrame ids and cancelAnimationFrame

Fixes https://github.com/web-animations/web-animations-js/issues/4
Merge pull request #245 from dstockwell/caf

Add requestAnimationFrame ids and cancelAnimationFrame
Merge remote-tracking branch 'web-animations-next/master' into HEAD
Add build artifacts from 52eeb92d040a916c59419f10875ed70ed66af371