fix DoVersion() when _SUBVERSION_{REVISION,DATE} are not set

Signed-off-by: Stefan Naewe <stefan.naewe@atlas-elektronik.com>
Improve interplay with marble.

The resulting gpx files can now be loaded and displayed by marble.
Writing the velocity to the gpx file.
udev rule + README update

The udev rule gives every user the right to access the GPS data logger.
Additionally the README also describes that the gpsd daemon might
also have been started by an udev rule and as such might be blocking the device.
Fixes testGPXTrackPTFormat4().
Fixes DoHelp() arguments problem.
Converts the README to Markdown.
Adds a pattern to the FindDevice function.
Adds *.gpx, *.nmea, *.pyc to the .gitignore.