Start of responsive styles.

@stephenminded 

Starting off this PR for responsive support in the assets framework for admin. Flex boxes make this sort of thing super easy.

**Basic concept**
![screen shot 2014-11-25 at 23 01 40](https://cloud.githubusercontent.com/assets/70303/5196185/7f00851c-74f7-11e4-8f9e-1083913aa9e0.png)
 
**TODO**

- [ ] Navigation dropdown toggle
- [ ] Fix spacing at the bottom of navbar
- [ ] Move sign out link somewhere?
- [ ] Handle long header text