Make header search less salient

Fades back the header search if it isn't in focus, to make it less salient on the page.

![screen shot 2014-10-07 at 4 46 34 pm](https://cloud.githubusercontent.com/assets/1239550/4537883/7eb0c16e-4de5-11e4-93fc-31a357a5e5ef.png)
![screen shot 2014-10-07 at 4 46 42 pm](https://cloud.githubusercontent.com/assets/1239550/4537884/7fc14fd8-4de5-11e4-9156-0ca2ea55f025.png)

closes #639