Stubbing out backend link for patients with additional IDs.
Remove undocumented location parameter from get_patient.
Link/unlink implementation in the dummy backend.
Django implementation of linking patients with ids. Simplify models.
Add support for link/unlink in the client API.
Usage docs on linking/unlinking patient IDs.
Bring back getting patients by id/source pair and document it this time.
Backend docs as well.
Tests for backend get by source id.
Django implementation of get by source id.
Dummy implementation of get patient by source id.
Client API test for passing source name to the backend on patients.get
Another small API test.
Minor fix to usage docs.
Merge branch '7-multiple-patient-ids'