CKAN metadata persistence (both row and column modes)

This PR (upon merging) will complete the #41 issue (except for the adjustments in MySQL and HDFS sinks sections regarding metadata persistence).

Tested:

* All unit tests pass
* Manually tested running CKAN sink in "row" mode, with notification-xml-md.sh and notification-json-md.sh, checking correct inserting in CKAN web front end for the Room1-Room resource (3 records each time)
* Manually tested running CKAN sink in "column" mode, with notification-xml-md.sh and notification-json-md.sh, checking correct inserting in CKAN web front end for the Room1-Room resource (1 record each time)
Both the cases the attribute hasn't metadata or an empty list of metada, a `[]` string is generated.