Split menu bar

We realized it would be nice if you could pair two menu bars together, and have one align to the left and one to the right, kind of like Foundation 5's topbar. This component does just that, through the magic of Flexbox.

- [x] Basic styles
- [ ] Mixins
- [ ] Finalize class names

What isn't clear is how this would become responsive. For now there won't be any JavaScript hooks with this thing; it's just a basic wrapper to do a split menu bar. We've talked about doing a topbar-esque directive for a future version of the framework, though.