Implement socket NMEA source.

nmeaSource property of PositionSource is limited to log files. This
change adds possibility of using a TCP socket instead. This is a lot
more useful when testing because it can be used with a simulator.

Change-Id: Ie6fb2b5344aae2702a8cf53d0146780d6ea1e0f9
Reviewed-by: Albin Olsson <albin.olsson@cybercom.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>