Add AsyncStringFeatureParser and use plain
XMLHttpRequest in the KML example

Since the content-type on github.io is application/octet-stream we need
to implement an Async string based parser interface in the KML parser.
Also use plain XmlHttpRequest in the example instead of jQuery Ajax since
the vector-features example also uses that.
Merge pull request #641 from bartvde/kmlexample

Add AsyncStringFeatureParser and use plain (r=@ahocevar)