Allow the use of sprite image in KML

This PR makes 2 things:

     - Add  the possibility to  specify an `offsetOrigin` in `ol.style.Icon` to use a sprite (see https://github.com/openlayers/ol3/pull/2040) 
    
     - Add parsing of `gx:x`, `gx:y`, `gx:w`, `gx:h` tags of KML file