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
Alphabetized Gemfile
Removed whitespace from users and sessions controller
Merge pull request #2 from CranAppleJuice/AD-add-monban-functionality

Added monban functionality to Rentr app