CKAN per column persistence

This PR implements issue https://github.com/telefonicaid/fiware-connectors/issues/60
This enumeration of fields is only for the row mode... I'll try to write something like:

Within files, Json documents are written following one of these two schemas:
* Fixed 7-field lines: recvTimeTs, recvTime, entityId, entityType, attrName, attrType and attrValue.
* A field per each entity's attribute, plus an additional field about the reception time of the data (recv_time). Regarding this kind of persistence, the notifications must ensure a value per each attribute is notified.