Move global nav to partial

![screen shot 2014-11-05 at 8 02 31 pm](https://cloud.githubusercontent.com/assets/3300569/4930758/c5a48128-6569-11e4-8f0f-61683ad74e9b.png)

- Move the global navigation to its own partial
- Refactor existing CSS for the nav

https://trello.com/c/7cYuLORT
There is a `modules/_modules.scss` file in which we might add the `_browser.scss` styles. Same with `_hero` below.