add photos_controller;
add listings_controller;
add deals_controller;
add index method to photos_controller;
(1) update relationship on model of photo;
(2) add resource routes for photos;
add index method to listings_controller;
(1) update relationship on model of listing;
(2) add resource routes for listings;
add index method to deals_controller;
add attr_accessor for user and listing on model of deal;
update relationships between models;
update the attr_accessor for listing on model of vehicle;