added devise modul for user login
added role management. there are 3 roles [admin, user (logged in), guest (not logged in]
admin can manage all
user can do everything except deleting cards
guest can only access cards#index and cards#show

roles are defined in models/ability.rb
to use permissions check you have to add authorize! [action] [element] to the specific function
e.g. def destroy
	@card = Card.find(params[:id])
	authorize! :destroy, @card
	...
     end