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
@NikitaAvvakumov I think its perfectly okay to have `nil`. `nil` isn't falsey! AFAIK `nil` acts as `false` everywhere except in equality.