Add a Sessions controller
Add tests for the new session action and view
Add a resource to get the standard actions for sessions
Add initial actions to the Sessions controller
Add initial signin view
Add tests for signin failure
Add tests for signin success
Add code for the signin form
Try handling failed signin
Add tests for signin failure error messages
Correct the code for failed signin
Complete the create action in the Sessions controller
Include the Sessions helper module into the Application controller

By default, all the helpers are available in the view but not in the
controllers. We need the methods from the Sessions helper in both places,
so we have to include it explicitly.
Add a test for the remember token
Add remember_token to the users table
Add a test for a valid remember token
Add a before_create callback to create remember_token
Add the sign_in function
Define assignment to current_user
Find the current user using the remember_token