* include/vaucanson/xml/node.hxx: Do not fill the alphabet with the
	default range anymore if an empty alphabet is clearly defined.
	Add the support for the alphabet ranges of the xsd file.
	* data/vaucanson.xsd: Change the default range from `ascii' to
	`implicit_alphabet', add a restriction for the range attribute.
	* doc/xml/xml_proposal.tex: Change the default range accordingly.