Move global nav to partial

![screen shot 2014-11-05 at 5 58 40 pm](https://cloud.githubusercontent.com/assets/3300569/4929907/e70d7a72-655a-11e4-9953-a32382fe313a.png)

- Move the global navigation to its own partial
- Refactor existing CSS for the nav
I changed the logo from an `h1` to a `header` tag based on some research I did online. A good article to talk about why it might be better to _not_ have your logo as the `h1` on your site here: http://www.fastcolabs.com/3016894/should-your-tag-be-your-logo