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.
I will try to explain this before the `event={body={the_json_part...},headers={{"content-type","application/json"}}}` part is described, because the Http header must be transformed into a Flume header within the event.