Initial KML support from GSoC 2013

This Pull Request includes most of the work I've done with the help of @mramato during Google Summer of Code 2013.

The items marked on the [roadmap](https://github.com/AnalyticalGraphicsInc/cesium/issues/873) are working and tested, some independent features like extrude need to be implemented before we can claim KML support, if you require more info please ask.