Ammends user login functionality when credentials do not match.
Adds logout feature

* User can logout and warden removes previously stored user.
* Adds helper methods in ApplicationController to track a current_user.
* Customizes warden behavior on how to store a User in session.
* Adds /logout route to clear a User session.
User registration feature

* A user can signup in the site.
* Adds dynamic_form gem to help displaying error messages on forms.
* RegistrationsController handles user sign up.