added inactivated_at to user's model
created scope and finder to deal with inactive users [12091751]
added custom finder to return a 404 when looking for inactive users [12091751]
used active_for_authentication? method to eliminate inactivated_users from login [12091751]
using deactivate instead of inactivate, sounds better
created deactivate method in user model [12091751]
removed unused lines from spec
Merge pull request #668 from diogob/deactivate_users

Deactivate users