Added monban functionality to Rentr app

* Used monban generators to created User and Session models and controllers
* Added sign in / up links to layout
* Added monban constraints: sign-in leads to dashboard page;
  sign-out leads to new session page
* Added "Rentr" dashboard link to layout
http://mrwgifs.com/wp-content/uploads/2013/08/I-Like-It-A-Lot-Jim-Carrey-In-Dumb-and-Dumber-Gif.gif