Add Relationship model, add user/relationship associations + tests
Add validations for presence of Relationship followed_id and follower_id
Add followed_users method to User model
Add follow! and unfollow? public instance methods to User model
Add followers public instance method to User model
Update sample_data rake task to include relationships
Add followers/following stats and follow unfollow forms
Add followers and following pages
Make the follow and unfollow buttons work
Add Ajax functionality to follow/ unfollow button
Add a first feed impelementation
Add subselects to promote scaling of feed