Many improvements to CKAN API support.

* [CKAN,NEW] Added several more predicates of the CKAN API v2.2.
* [CKAN,EXT] Added legend for groups and users.
* [CKAN,JSON_TO_RDF,EXT] Added primary key for legends (optional), used for generating URIs.
* [CKAN_TO_RDF,EXT] Use newly added CKAN API predicates.
* [JSON_TO_RDF,EXT] Skip empty atoms (not asserted as literals).