Add Reservation to Rentr

* Users can create reservation dates
* Users can not create invalid reservation date ranges
* Users can view their reservations for a particular listing
* Users can view all of their reservations
* Users can view all their reservations for a listing if any exist
* Guests can still use app
[`link_to_unless_current`](http://api.rubyonrails.org/classes/ActionView/Helpers/UrlHelper.html#method-i-link_to_unless_current) ? :smiley: