Replace password_digest with password and password_confirmation in the factory.
Add authenticate method.
Create admin/sessions controller.
Add current_user method and set it as helper method.
Add nav_user helper method.
Show nav-user div on the nav-bar.
Adjust the css style of nav-user.
Add the authorize filter for all admin controllers.
Create admin_controller.
Set the before_filter in admin_controller.
All admin controllers inherit admin_controller so that all actions will
be affected by the filter.
Update the controller specs because of the filter.