This adds ol.parser.ogc.KML which can be used to read and write KML documents.
NetworkLinks are retrieved asynchronously.
Current caveats of the implementation are:
* LabelStyle not yet implemented. Missing support in renderers.
* When using shared structures the parser needs to be configured with
dimension 2.
* We need a better way to disable fill, currently we use opacity as a
workaround.
* We cannot really roundtrip documents, since some of the info is not
preserved in the ol structures. But we can write out most of the important
info.
Parsers that read features from strings, objects, or dom elements should implement the right interface. Among the read options is a callback that gets called after feature construction and before geometry construction. This callback is called with the feature and the geometry type and returns a shared vertices structure. The geometry is then constructed with this shared vertices structure before being added to the feature.
