Filter by role

www.agileplannerapp.com/boards/173808/cards/4223

giving the ability to admins to filter users list by role,
so that they can easily identify admins and superadmins
and restrict users from having unnecessary privileges.

![screen shot 2014-07-07 at 12 59 31](https://cloud.githubusercontent.com/assets/230074/3492361/893bf73c-05a8-11e4-8a69-eb519730fc31.png)