Animation loop should use or reference requestAnimationFrame

[zachberry]

The loop code uses setInterval which will work in every browser, but we should at least mention requestAnimationFrame which is more efficient and built for animations.

More info: http://paulirish.com/2011/requestanimationframe-for-smart-animating/

[zachberry]

Interesting debate on requestAnimationFrame: http://stackoverflow.com/questions/7999680/why-doesnt-jquery-use-requestanimationframe

My experience with requestAnimationFrame has just been limited to a tiny amount of three.js work where I didn't come across these problems - regardless, it's worth looking at and perhaps mentioning to some degree if requestAnimationFrame is added into the tutorial.