11.1 Add models for follower/followee relationships.
11.1.2: Added relationship associations (models) between microposts and users
Added validations for presence of follower and followed id in relationship model
11.1.2 revision: Don't allow access to follower_id in Relationship model.
11.1.4: Added followed_users functionality to user model.
11.1.5: Added followers functionality to user model (through reverse relationship).
11.2.1: Populate sample relationships.
Add following/followers stats to home page and user pages, and follow/unfollow buttons to user pages. Note that controllers do not handle follow/unfollow buttons yet.
11.2.3: Show following/followed users.
11.2.4: Enable follow/unfollow buttons to work by adding relationshipsController.
11.2.5: Switch to AJAX requests for follow/unfollow buttons.
11.3.{1,2}: Add user feed functionality to user model.
11.3.3: Use SQL subselect for feed to optimize/scale for users with lots of followees.
11.5.1: Test that relationships are destroyed upon user destruction.
11.5.2: Changed respond_to to respond_with in RelationshipsController.