Listings have a Room Category

* Create migration for `room_categories`
* Create `RoomCategory` model
* Add default `room_category` seed values (`rake db:seed`)
* Add uniqueness validation to `RoomCategory` and `PropertyCategory`
* Define relationship to `RoomCategory` in `Listing` model
* Add `room_category` drop down to `listings#new` form
* Add `room_category_id` to new listings in `ListingsController`
* Show room category on `listings#show`

https://trello.com/c/XRDuqmFz