Paranoid requestAnimationFrame for antique Firefox

Very old versions of Firefox - e.g. Firefox 7 - have
window.requestAnimationFrame but not cancelAnimationFrame. This is a
very old release so the easiest fix is simply to check for both of the
functions which we intend to call and fall back on traditional behaviour
if both aren't present.
Merge pull request #103 from acdha/paranoid-requestanimationframe-polyfill

Paranoid requestAnimationFrame for antique Firefox