LUN-915 -- Ability to assign roles to non-staff users as well.

When user is assigned a role, he implicitly becomes a staff user.
This does assume that all roles are built for staff users.
LUN-917 -- Add user search by using the 'chosen' jquery plugin (http://harvesthq.github.io/chosen/)
LUN-917 -- Implement user search within the current site
Merge pull request #11 from pbs/LUN-915

LUN-915 & LUN-917 -- Increase usability by adding user filter boxes