navigation enhancements

* all dropdowns are left aligned
* entire width of li is now selectable in user, search, and category dropdown (needs further enhancement for notifications)
* background color hover state 

![screenshot 2014-06-03 21 56 36](https://cloud.githubusercontent.com/assets/1681963/3168860/4af2ae48-eb8c-11e3-9c19-291221e64960.png)


:sunglasses: