Add buddies

https://trello.com/c/bjJNY4Ub/44-users-can-friend-each-other

Some latent code was commented for next PR
A few things:

* I would name this `self.for` instead. Then you can do `BuddyRelationship.for`, which is like "the buddy relationship for these people".

* Can you come up with better variable names than `user_id_one` and `user_id_two`? What are their roles?