KML: Add parsing of <gx:Track> in <Placemark>

Add parsing of of <gx:Track> in <Placemark>

Example of file using this: https://drive.google.com/file/d/0B4uRuJ1hajZmc1U4VTh2RXEzcDg/edit?usp=sharing

This PR add tests for parsing <gx:MutliTrack>  and <gx:Track>