Add verbatim copies of the GPX / TCX extension XSDs

Just for unit tests; copied verbatim from:

* http://www.cluetrust.com/Schemas/gpxdata10.xsd
* http://www8.garmin.com/xmlschemas/ActivityExtensionv2.xsd
* http://www8.garmin.com/xmlschemas/TrackPointExtensionv1.xsd