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.
CKAN still using iso8601data  due to that implications in the code you mention (I preferred to have the PR stick to the topic of multi-tenancy). However, I plan to do a overall refactoring of name changes in the next one, also affecting to MySQL and HDFS sinks to move literals to constants (if you agree of course... not sure if it would clash with ongoing work in those part from your side).