Support the DOM tree generated by the node.js package xmldom

I couldn't get jsdom to work, and it really seems to be the wrong library for parsing plain RDF/XML in a node.js environment as it pretends to be a full browser environment.  xmldom worked much better, but the patch below is necessary to parse the DOM tree it returns.

It could be a good idea to replace jsdom in rdflib.js with xmldom.  Usage is easy, a direct parallel to the browser DOMParser:

    var doc = new xmldom.DOMParser().parseFromString(xml, 'text/xml');

(I can do that patch too, but don't have time this week.)