Update user model, create users controller, add new crete actions.
Create new view in users views directory.
Adds to user model to make sure user must enter email, password. only users with both may be created.
Creates route for new session, sessions controller, new session view.
Create login and signup route, correct user authenticate error.
Creates songcollectsions model and controller, add has many through to user model for songs, updates create method so a current user can add a new song to her collection rather than all songs.