Multi tenancy in REST handler and CKAN sink

Implements #35 in the REST hander and CKAN sink.

Tested:
* Existing unit test pass (although no new unit test has been added)
* Manual testing with the notification-json-simple.sh script in three different situations: 1) CKAN organization doesn't exist, 2) CKAN organization exists but default package doesn't exist, 3) CKAN organization and default package exist.
As discussed offline, this class will be moved to CKANBackendImpl.