Add preliminary support for c:geo export GPX (including groundspeak extension)

[mrummuka]

c:geo (http://www.cgeo.org/) is open-source application for geocaching, that supports exporting geocaches into gpx format. However, the gpx it generates is 1.0 compatible format, but with additional elements within waypoints.

This pull request adds support for parsing the following c:geo generated elements from waypoints:

<url>
<urlname>
<sym>
<type>
<groundspeak:cache id="XXX">
<groundspeak:cache><name>
<groundspeak:cache><placed_by>
<groundspeak:cache><type>
<groundspeak:cache><container>
<groundspeak:cache><country>

However, this pull request does not change the original behavior, but only parses the elements if it recognizes that the 'creator' attribute for element 'gpx' contains "c:geo - http://www.cgeo.org/"