Introduces ButtonGroup

:warning: This PR is out of date, since the `EM` based sizes landed.

Multiple `Button`'s can be wrapped with `<span class="digit-ButtonGroup">` to group them together.

![screen shot 2013-10-30 at 12 26 05 am](https://f.cloud.github.com/assets/378023/1429896/ceeaf24c-40b9-11e3-85a9-370f4e10926c.png)

It's not really a new component, just some styles part of `Button`. We're doing it the same in Matte. But it feels a bit strange like that, maybe at some point it could be its own component. We could also add some more features like:

- Support for toggling the buttons
- Support for only one active (like a radio button group).