Demonstrates parsing of gx:Track elements from KML.
                If a KML document contains <gx:Track> 
                elements and the extractTracks property is set true on the 
                parer, features will be created that represent track points.
                Each feature will have a when attribute that contains the 
                value of the relevant <when> element from 
                the track.
            
View the kml-track.js source to see how this is done.