mvc framework required to implement sponsor_osra_id

part of PT story https://www.pivotaltracker.com/story/show/75999516

- 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
- added sequential_id field and osra_num field to sponsor table
- implemented tests for osra_num functionality
- implemented osra_num functionality

`^` works as a bitwise operator on numbers but as a binary XOR on booleans (as I just now learned), so using here is most appropriate. Great call, @motasem-salem :clap:
![screen shot 2014-09-24 at 3 04 13 pm](https://cloud.githubusercontent.com/assets/5029403/4387844/f329b120-43e2-11e4-9095-50c542657ae4.png)