Create shift scaffold
Update form for editing shift.
Make coop selection implicit, make views show coop name, not ID.
Added coop name to swaps
Allow user to elect themselves as leader of shift.
Super commando => commando
Associate shift with coop.
Create HABTM relation between Users and Shifts.

TODO: take advantage of this in user/shift controllers.
Allow users to join and leave existing shifts.
Fix missing template bug in shifts_controller#remove_user.
Remove duplicate reference to coop in user model.