Add sessions controller
Add tests for sessions
Add paths for signin and signout
Add new, create, and destroy actions for sessions
Add view for sign in
Add test for invalid signin
Add test for valid signin
Add sign in form
Add authentication of signin
Flash error for invalid signin
Add test for errors only showing on signin page
Only flash errors on signin page
Add test for presence of remember token
Add remember token to User model
Add test for remember token
Add remember token function for User model
Add sessions helper to application controller
Add sign in function
Add current_user functions
Add signed_in? function