User authentication

Add ability for users to log in and log out. Resolves #10 
Not a huge fan of including view helpers in the controller.  Prefer to go the other way with defining methods in the controller (or another module in `lib` to be included into the controller) and using [`helper_method`](http://api.rubyonrails.org/classes/AbstractController/Helpers/ClassMethods.html#method-i-helper_method) if it needs to be used in the view.