Users can create a listing

https://trello.com/c/2Ma1JSBO/3-as-a-user-i-can-list-a-room-apartment-house

![image](https://cloud.githubusercontent.com/assets/8646429/4981664/2bdf780e-690b-11e4-870e-651855627d44.png)

Usually when argument lists are this long, we put each on their own line:

```ruby
params.require(:listing).permit(
  :title,
  :price,
  :location,
  ....etc
)
````