Design update to header to match next-gen style

Design update from Ben W to make the header links match next-gen style. Lower case-text, heavier font-weight, and a minor alignment fix.

Before:
![screen shot 2014-12-30 at 12 59 03](https://cloud.githubusercontent.com/assets/123386/5578702/491bc400-9024-11e4-817d-87da6c90798e.png)

After:
![screen shot 2014-12-30 at 12 59 12](https://cloud.githubusercontent.com/assets/123386/5578704/4dcbd314-9024-11e4-83b7-b37a14922ebc.png)