Polyfill for requestAnimationFrame and cancelAnimationFrame.
WARNING: this version of the code is only lightly tested. I'll remove this warning after it's been run on a variety of targets.
Adapted from https://gist.github.com/paulirish/1579671 which derived from http://paulirish.com/2011/requestanimationframe-for-smart-animating/ http://my.opera.com/emoller/blog/2011/12/20/requestanimationframe-for-smart-er-animating
requestAnimationFrame polyfill by Erik Möller. Fixes from Paul Irish, Tino Zijdel, Andrew Mao, Klemen Slavič, Darius Bacon
MIT license.
To do: incorporate improvements from http://stackoverflow.com/questions/13241314/up-to-date-polyfill-for-requestanimationframe and https://gist.github.com/mrdoob/838785