Add Property Types

* Create model for `PropertyType`
* Create migration for `property_types` table and add ref in listings
* Add seed data for property types (populate with `rake db:seed`)
* Add property type drop-down to `listings#new` form
* Update controller to add `property_type_id` to new listings
* Add property type to `listings#show` view
I might have named this `PropertyCategory` instead. When I see something named `_type` in the database I automatically think of polymorphic associations, so I try to avoid using it.