add form to site-header navbar

![screen shot 2014-10-06 at 5 00 11 pm](https://cloud.githubusercontent.com/assets/1239550/4521924/4edae49a-4d1f-11e4-9f3d-fff916de2ca3.png)
![screen shot 2014-10-06 at 4 59 58 pm](https://cloud.githubusercontent.com/assets/1239550/4521923/4ed92da8-4d1f-11e4-93ab-b9e6bfc74c4f.png)
![screen shot 2014-10-06 at 5 09 05 pm](https://cloud.githubusercontent.com/assets/1239550/4521925/5216b0b2-4d1f-11e4-9118-e124399c68dd.png)

I think we should make the wide screen version less salient. If we merge this as I'll add a ticket for making this less salient on wide screens. It could look like the other nav items then expand to the form appearance when focus is in it.

closes #310