Add an xml format for a label node, part II.

	* include/vaucanson/algebra/implementation/series/rat/xml_exp_visitor.hh,
	* include/vaucanson/algebra/implementation/series/rat/xml_exp_visitor.hxx:
	New. Add an exp visitor that gives back the xml tree of an expression.
	* include/vaucanson/algebra/implementation/series/rat/exp.hh
	* include/vaucanson/algebra/implementation/series/rat/exp.hxx:
	Add definition of the visitor, called `xml_tree(<node_to_create_name>)'.
	* include/vaucanson/xml/XML.hh,
	* include/vaucanson/xml/XML.hxx: Add new boolean to decide wether the
	xml output of the label is an attribute or a node.
	* include/vaucanson/xml/tools.hxx: Add `add_xml_label' procedures that
	give the labels of an automaton as nodes in the XML output.
	* include/vaucanson/xml/xml_chooser.hh,
	* include/vaucanson/xml/xml_chooser.hxx: Launch `add_label' or
	`add_xml_label', according to the boolean value.
	* include/vaucanson/xml/xml_converter.hxx,
	* include/vaucanson/xml/xml_converter.hh: Update to allow the changes to
	work.
	* src/tests/xml/xml_label_node_tester.cc: New. Add a test of the `node'
	version of an XML label output.
	* src/tests/xml/tester.sh: Add the label_node test.
	* src/tests/xml/label_node_ref.xml,
	* src/tests/xml/label_node_ref.dot: New. Add reference files.
	* src/tests/xml/Makefile.am: Update accordingly.
	* src/tests/xml: Add tmp files of the test to svn ignored files.