Add support for gpx_file in OSM v0.6 API classes

[HarelM]

I want to update Osm trace using the OSM API and OSM Sharp or to get a trace data.
The following is the response that is returned from an API call:
API address:
https://www.openstreetmap.org/api/0.6/gpx/2463219
Response:

<osm version="0.6" generator="OpenStreetMap server" copyright="OpenStreetMap and contributors" attribution="http://www.openstreetmap.org/copyright" license="http://opendatacommons.org/licenses/odbl/1-0/">
<gpx_file id="2463219" name="20170726.gpx" lat="42.6896" lon="-84.4321" user="jojkuhiuwer" visibility="identifiable" pending="false" timestamp="2017-07-26T21:04:01Z">
<description>NU3001</description>
<tag>GPSLogger</tag>
</gpx_file>
</osm>

I'm not sure, but I don't see that this type of response is defined in the Osm class, right?

[HarelM]

@xivk, I have created the relevant pull request.
Please let me know if you want me to add anything else.
If not, can you please create another NuGet package with these changes?
Thanks!

[HarelM]

P.S. I wasn't sure whether Visibility should be an enum or not...
If it's not an enum the interface is more flexible but also more error prone... your call...

[HarelM]

Closed via f83d305