Animated Digit

Added some more transitions:

![digit-animated](https://cloud.githubusercontent.com/assets/378023/3451558/3896ab80-019b-11e4-8951-814cef03edc0.gif)

:warning: This isn't finished yet. Just submitting to see if there is enough interest/priority. Otherwise we can close it again. Some things that still need fixing:

- [ ] RadioButton and Checkbox icons don't animate on iOS. Maybe a bug with scaling SVG.
- [ ] If a RadioButton or Checkbox are initially checked, they transition, but shouldn't. Maybe it's because the `montage-Checkbox--checked` gets added after the element is already added/rendered to the DOM, triggering the transition?