add 1-to-many relationship between user and listing;
add 1-to-1 relationship between vehicle and listing;
add attr_accessor for user and vehicle on model of listing;
(1) update attr_accessor for vehicle on model of photo;
(2) update relationship between photo and user as well as between photo and vehicle;
add attr_accessor for photo on model of user;
add attr_accessor for listing on model of user;
minor update on add attr_accessor for vehicle and photo on model of user;
add attr_accessor for photos on model of listing;
(1) add attr_accessor for photos and listings on model of vehicle;
(2) add dependent to vehicle on its relationship with listing;