Request animation frame

We can get much better performance and redraw speed by using `requestAnimationFrame` instead of timeouts in a tight loop.

This PR is on top of #18, so it should be merged first (sorry, @benschwarz but I’m being lazy and didn’t want to have to deal with merging the two later)

### TODOs

- [x] Add a polyfill for rAF (https://gist.github.com/paulirish/1579671)

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#### Before
![before mov](https://cloud.githubusercontent.com/assets/18076/3348195/66733958-f91a-11e3-999a-a37163f3ead8.gif)

#### After
![after mov](https://cloud.githubusercontent.com/assets/18076/3348205/12842194-f91b-11e3-993c-6e52c0c229fc.gif)