make sure only admin can delete group in controller

is this what you mean @PragTob ? 
closes #206 

(i'm cleaning it up a bit thou first...)
This before somewhat duplicates the before from the other context... you could move that before up a level so that it is executed for both contexts and then in this before here just do `person.add_role :admin`. I realize that what I just wrote might be confusing so we could do it on the next meeting - no biggie now :)