Add keyboard shortcuts to header; Add group dropdown search capability

A couple simple keyboard shortcuts for the navigation bar; 

Shortcuts are
- 'n' for opening the notifications dropdown
- 'g' for opening the groups dropdown
- 's' for focusing the search box
- 'u' for opening the user dropdown (profile, sign out, etc.)
(open to more suggestions)

also adds a search box to the groups  dropdown if the number of groups is > 15 (which is about height where it starts scrolling.)

Screenshots!
![screenshot from 2014-07-30 10 02 51](https://cloud.githubusercontent.com/assets/750477/3750078/46b84a94-17f2-11e4-9624-31a2190cfb96.png)
![screenshot from 2014-07-30 10 03 00](https://cloud.githubusercontent.com/assets/750477/3750077/46b7c538-17f2-11e4-9754-67cb5284ae84.png)


Todo:
- Make search groups input case-insensitive
- Write a test for the keyboard shortcuts (it's harder than it should be to simulate keyboard input in Cucumber)

Take a peek! @robguthrie @rdbartlett @joshuavial