Allow libxml.Document and libxml.Element to be passed to xmlconv()

This makes it possible to convert sub-elements of a document that has already been parsed (using xpath or child-getters, etc).

I couldn't figure out tap, but this change allows something like this:

    fs.readFile('test/data/media.xml', 'utf8', function(err, xml){
	  var doc = libxml.parseXml(xml);
	  var element = doc.get("//book[@id=1]");

	  var obj = xmlconv(element, {convention: 'parker'});
	  console.log(JSON.stringify(obj, null, '  '));
    });

produces:

    {
      "title": "XML: The Good Parts",
      "author": "Nulon Catorze",
      "blurb": "\n      This is an awesome book, you guys!\n    "
    }