Add user authentication

* Add monban and monban-generator gems to Gemfile
* Include Monban::ControllerHelpers in ApplicationController
* Generate monban:scaffold
* Break sign-in and sign-up forms out into partials
* Establish root route as homes#show
* Create homes_controller with show action
* Create homes#show view which renders the sign-in and sign-up forms
And maybe not this one.