[vector-api] KML LinearRing support

[KML supports LinearRings as geometry type](https://developers.google.com/kml/documentation/kmlreference#linearring), but ol3's geometry model is based on GeoJSON which [does not have an explicit LinearRing geometry type](http://geojson.org/geojson-spec.html#linestring).

This PR adds support for KML LinearRings by reading them as Polygons.