[vector-api] KML gx:MultiTrack support

This PR adds support for `gx:MultiTrack` geometries. It also demonstrates the `vector-api`'s correct handling of XML namespaces (cf #1406).
![kml-gx-multitrack](https://f.cloud.github.com/assets/6942/1791413/cac50b02-698c-11e3-86c5-6eeeba491769.png)