Header refactor

# Changes
 - Adds `.-centered` header pattern modifier, to be used on new 404 page.
 - Some minor cleanup to header pattern.

# Screenshots
#### Desktop example:
![screen shot 2014-10-14 at 5 37 50 pm](https://cloud.githubusercontent.com/assets/583202/4637337/5c624392-53eb-11e4-8842-94b6d87885f7.png)

#### Mobile example:
![screen shot 2014-10-14 at 5 42 13 pm](https://cloud.githubusercontent.com/assets/583202/4637340/63b0b9f8-53eb-11e4-96fb-ac6f2e8a7f2c.png)