Add a parser for reading and writing KML.

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.