[WIP] Tightening up the visual design.

I'm using this PR to track groups of related changes to do a pass through transonic, bringing a little consistency to the visual design and bringing it closer in line to pwlam's lovely style guide.

To keep this from being a single, monotlithic commit of only tangentially-related changes, I'll be breaking this up into a number of different commits, updating this pull request with progress and screenshots as I go. We can rebase before merge if that's desirable.

## Table of contents

1. Update design of navigation

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### Update design of navigation

![nav](https://cloud.githubusercontent.com/assets/23885/3475220/0238b25a-02ee-11e4-8b46-05ed85d9c2ff.jpg)

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Alright, if we change the whole transonic to look like opdash mocks, it'll be easier to copy/paste. I'm good with that.