User can request on other user's listing.

* Generate Request model

![request](https://cloud.githubusercontent.com/assets/3759510/5001775/89441d58-69c7-11e4-93e2-1eca81c722a2.gif)

In the RANGE (1..10) - perhaps we could include a column when a user makes a Listing called "max-guests" that you can reflect in the range. 

Range would be something like:        in: 1.. listing.max_guests, value: 1