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.
If you take a look to the other sink descriptions, you'll see the sink is explained through the notification example appearing at the begining of the major section "Functionality explained (Json notification example)". Thus, you should try to describe how the data is persisted in the datastore, both in the row and column persistence.