Proposals  belong hubs Schema change

Not sure this is ready for merging, as it contains only the migration and model changes, no code adjustments.

 - While the hub_id is present on Proposals, for testing hub_id has not yet been removed from Votes.

 - If someone wants to begin working on the code adjustments please feel free to merge or pull the branch, and I will begin working on the seed file to populate data for testing.

![Proposal belongs to Hub Schema](https://f.cloud.github.com/assets/2165336/88154/6ee2df64-64e0-11e2-97e2-3991a4621f4a.png)