Add horizontal padding classes to header

Hey there!
Awesome work, I really love it! :+1: 

There was just one thing that was bugging me, the header was leaking out to the sides in the middle breakpoint, so here is a little fix.

__Before:__
![screen shot 2014-08-27 at 11 01 55 pm](https://cloud.githubusercontent.com/assets/2394509/4066599/52adb944-2e25-11e4-835a-49b880d06fe0.png)

__After:__
![screen shot 2014-08-27 at 11 02 15 pm](https://cloud.githubusercontent.com/assets/2394509/4066604/5a78d974-2e25-11e4-9dfa-1e49f9b0703f.png)

Thanks! :smiley: