Users can sign up and sign in

* Add `monban` and `monban-generators` to Gemfile
* `rails g monban:scaffold`
* `rake db:create db:migrate`
* Require users to login for all controller actions by default
* Add "Sign In," "Sign Out," and "Sign Up" links to application layout
* Add h2 heading to sessions#new and users#new forms
* Remove comments from routes file

Sign up: https://trello.com/c/gDbmHH1Z
Sign in: https://trello.com/c/wfjliFVH
Users can perform guest actions: https://trello.com/c/cd2YGjlL