Add organization model

Added a model for organizations, and started saving it to the datastore in run. Addresses issue #19.

* [x] Add organizations to model.
* [x] Add organizations to `run_update.py`.
* [x] Expose organizations in the API.
* [x] Decide whether to call them “brigade” or “organization”.
* [x] Link Organization model to Project model.
* [ ] Merge forthcoming numeric ID work from master.