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
I do not agree because I think the seed command should not be a part of this. `db:setup` includes `db:seed`. Instead it should be only till the migration step.