add support for invalid but common URL tag inside of NetworkLink KML

[mmacaula]

When exploring KML support in Cesium, we keep running across invalid KML in the 'wild' which uses a Url tag name instead of the standard link. Google Earth accepts this invalid KML as well.

 <NetworkLink id="link">\
    <Url>\  <!-- instead of Link --> 
      <href>./Data/KML/refresh.kml</href>\
    </Url>\
  </NetworkLink>

This is a fix that will allow those types of KML to be parsed. It should expand the range of KML that Cesium can accept.

[mramato]

Thanks @mmacaula, I confirmed that Google Earth works as described and accepts the Url node in place of Link. I'll update CHANGES in master.