* data/vaucanson.xsd: Change the structure of the label as a node. The
	element corresponding to the star is now a node instead of an attribute.
	* include/vaucanson/xml/tools.hxx: Change the parsing of an XML node
	accordingly.
	* include/vaucanson/algebra/implementation/series/rat/xml_exp_visitor.hxx:
	Change the `star' case of the xml visitor. A star node is now created.