Tightening up the visual design.

I'm going to be breaking this into a bunch of smaller commits, using this PR to keep running track of this work and updating the description with screenshots as I go.

---

### Update design of navigation

![nav](https://cloud.githubusercontent.com/assets/23885/3475220/0238b25a-02ee-11e4-8b46-05ed85d9c2ff.jpg)

---
that's way too big. we need room for the previews.