Improve previews for XML based resources

The old data previewer code used to call the dataproxy with XML-based resources to at least show the source code between pre tags [1]. The new code just loads the remote resource on an iframe, so the rendering is left to each browser. On chrome is not that nice [2].
XML-based resources are widely used (eg Linked Data formats like rdf-xml, rss, etc) so it may be nice to display them consistently.
There are different options for a potential xml_preview extension, from just showing the code between pre tags as previously done (we could use resource_proxy instead of dataproxy now) to use a syntax highlighter to render the XML.


[1]
![Old](http://i.imgur.com/qFWZu.png)


[2]
![New](http://i.imgur.com/1163O.png)