Firefox support (and possibly Safari)

The game wasn't working in Firefox due to `window.requestAnimationFrame` being undefined. It turns out that Firefox uses a prefixed version, `mozRequestAnimationFrame`. This code checks for the prefixed version and uses that if unprefixed isn't defined, which is enough to get the game working on Firefox 22.0a2. I also check for `webkitRequestAnimationFrame` which may (or may not, I haven't tested) get Safari 6.0 working.