Taggin documents during bulk csv import 

Pull request I'm issuing concerns tagging documents during csv import. Our main use for Nuxeo Platform is for data archival, which means importing a large quantity of existing files. It is imperative for us to be able to set all necessary metadata for the files during import, this includes tags.
Since nuxeo-csv plugin did not have the ability to perform documents tagging during import I modified the code to support this feature. 

Apart from that I realized that certain metadata could not be set during import (such as "creator" or "issue date") and they were filled with default values. I modified the code (my second commit) so that not it is possible.