mvc framework required to implement sponsor_osra_id

part of PT story https://www.pivotaltracker.com/story/show/75999516

- *** This does not create a sponsor_osra_id yet ***
- model with validation that 1 and only one branch or oganisation is selected
- migration to add organization reference to sponsor
- belongs to organization reference in model
- specs to check 1 branch or orgnaization has been selected
- fixed sponsor factory so it is valid against new criteria
- fixed feature files to  reference a branch so they can be valid
- changed active admin form to show the organization and branch drop downs
How about 
```
def belongs_to_one_branch_or_organization
    unless branch.blank? ^ organization.blank?
      errors.add(:organization, "must belong to either a branch or an organization")
      errors.add(:branch, "must belong to either a branch or an organization")
    end
  end
```

It's shorter but if you think it's less readable or maintainable, then let's keep it as is.