Add schema for extension elements

[csarven]

compass and compass_accuracy (and maybe others?) are currently using the gpx namespace: http://www.topografix.com/GPX/1/1 . The gpx schema however says the following for extensions:

You can add extend GPX by adding your own elements from another schema here.

(see also http://www.topografix.com/GPX/1/1/gpx.xsd )

It doesn't really matter what/where that schema is. The schema URL doesn't need to resolve either but somewhere reliable long term would be useful to look it up, or at least an intention to create it later it. A wiki entry on github will sort of do, but that might not be persistent in the future (especially if the definition is to be relied on). Something to consider...

[icrf2000]

I stumbled over this too. Have a version (that I consider for creating a pull request) which exports the tags into the GPX file. This is also an matter. For sake of consistency with the existing code, I did it without namespace, but actually this and also the elements csarven mentions should be using a separate namespace.

[Binnette]

Related to #330

[Binnette]

This need design.
The bug just say "we need to support in GPX file".
What is your usage/need? What field you want to add? Accuracy? direction? What else?