'Returns an xml string representing this module, and all modules
underneath it.  May also write required resources out to resource_fs.
Assumes that modules have single parentage (that no module appears twice
in the same course), and that it is thus safe to nest modules as xml
children as appropriate.
The returned XML should be able to be parsed back into an identical
XModuleDescriptor using the from_xml method with the same system, org,
and course'

def export_to_xml(self, resource_fs):

    node = Element(self.category)
    super(XmlDescriptor, self).add_xml_to_node(node)
    return etree.tostring(node)
