Add OSRA Branches

PT Story: https://www.pivotaltracker.com/story/show/77972984

- [x] Added Branch model and seed data
- [x] Added Branch specs and factory
- [x] Added relationship between Sponsor and Branch
It seems we don't have an agreement so far. I will not modify this at the moment in this PR, but I'll keep the question #28 open.

If we decide to do something about db constraints later, we can tackle that in another PR.