Implement Document.createElementNS()

This is a proposed implementation for Document.createElementNS(). One reftest is included.
Disclaimer: I'm not sure this is the best way to do it but I wanted to give it a try. In particular, Servo is very html-centric for the time being, so I introduced a new nonHTMLTypeId for everything non-html for the time being. Of course, that will have to be derived into svg and other typeids when needed...