Saving a mapping of MMSI -> ShipName to CSV file

This makes AIS position reports immediately useful upon app open, before a static packet is received for the vessel.

Many of us sail in the same geography and see many of the same vessels repeatedly. It's nice to have those vessels identified by name immediately upon receipt of the first position report.

See also discussion here: http://willkamp.com/opencpn/flyspray/index.php?do=details&task_id=1246