fix display of masthead in Safari

Roberta, by removing `position: fixed` from the `.nav` and `.brand` class, the navigation displays, but is a little smaller in Safari.

![screenshot 2014-09-24 19 50 10](https://cloud.githubusercontent.com/assets/228373/4397602/8f4a8fae-4445-11e4-9ad5-b542e2ef7719.png)

If you "merge" this change into github, it will show up.

Once you "merge" this change, be sure to sync your local project using the Github application.

I'm going to make some additional changes to you css, so that you can see how I would structure it.