Header nav sizing

Tablet width:
![screen shot 2014-04-01 at 5 49 35 pm](https://cloud.githubusercontent.com/assets/583202/2585265/010d8222-b9ea-11e3-8930-d18c56fe2368.png)

Desktop width:
![screen shot 2014-04-01 at 5 49 48 pm](https://cloud.githubusercontent.com/assets/583202/2585268/07627254-b9ea-11e3-974c-383809904b95.png)

Adjusts spacing on nav items; adjusts font-size based on new 18px base size. Adds a min-width on logo container to keep spacing comfortable on tablet (downside: breaks first nav item off of the grid).