Added XML builtins

The only function available now is toJSON() to convert an XML doc to a JSON object.

Test case:

~~~
var json = xml.toJSON("<animals><dog><name>Rufus</name><breed>labrador</breed></dog><dog><name>Marty</name><breed>whippet</breed></dog><cat name='Matilda'/></animals>");

Ti.API.info("JSON: " + JSON.stringify(json));
~~~

Result would be the JSON formatted XML document:

{"animals":{"dog":[{"name":"Rufus","breed":"labrador"},{"name":"Marty","breed":"whippet"}],"cat":{"name":"Matilda"}}}