Use Monban for authentication

* Add Monban to Gemfile
* Add password_digest field to Users
* Include Monban helpers in ApplicationController
* Add password fields to login and sign up views
* Update UsersController and SessionsController to use Monban methods
Add home page with sign in / sign up forms

* Created homes#new
* Extracted sign_in_form to partial
* Extracted sign_up_form to partial
Update monban to version 0.1.1
Require login before accessing galleries