Sort user listings in admin view by approved status

#### Note ####
- sorts user listings by <code>approved</code>, such that unapproved users are at the top, and then sorts the respective groups alphabetically based on email address (original sorting criteria)

#### Screenshot ####
![screen shot 2014-04-03 at 4 19 28 pm](https://cloud.githubusercontent.com/assets/456952/2608661/4c7ae716-bb6d-11e3-844b-e9e8b474b6b3.png)