Using requestAnimationFrame for improved perfs

Use recent browsers' support for the requestAnimationFrame API will prevent superfluous window repaints and therefore increase the overall performance of the page.

## Some background

- [Leaner, Meaner, Faster Animations with requestAnimationFrame](http://www.html5rocks.com/en/tutorials/speed/animations/)
- [Rendering: repaint, reflow/relayout, restyle](http://www.phpied.com/rendering-repaint-reflowrelayout-restyle/)