Updated JXON to match latest Mozilla code

This mostly just adds the `sNamespaceURI`, `sQualifiedName` and `oDocumentType` parameters to `JXON.unbuild` (and `JXON.stringify`), which makes it possible to serialize objects into XML documents with namespaces (though *actually* supporting namespaces required a few additional changes to the Mozilla code.)

I also made `JXON.unbuild` handle objects with `@xmlns` properties sanely.

I'd appreciate it if you could bump up the version number and publish the new version on npm ASAP if you accept this pull request, so I can just pull it as a dependency in my code.