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.
This is mainly an utility class for CKANBackenImpl class. Not sure if it should be in a separate file (as now) or as a embedded class? (not sure of the name) of CKANBackImpl. What do you think?