This patch re-activate XML session support. Usage is the same as 
	before, but the class is named xml_session.
	
	* include/vaucanson/xml/session.hh,
	* include/vaucanson/xml/session.hxx: New. XML session support.

	* include/vaucanson/xml/xml_converter.hh,
	* include/vaucanson/xml/xml_converter.hxx,
	* include/vaucanson/xml/xerces_parser.hxx,
	* include/vaucanson/xml/tools.hh,
	* include/vaucanson/xml/tools.hxx: Code maintainance to ease XML 
	session handling.

	* src/demos/xml/make_session.cc,
	* src/demos/xml/display_session.cc: New. Re-add demos for session. 
	These demos are temporarely limited to boolean automata. 

	* src/tests/xml/trans_ref.xml,
	* src/tests/xml/r_ref.xml,
	* src/tests/xml/z_ref.xml,
	* src/tests/xml/z_min_plus_ref.xml,
	* src/tests/xml/b_ref.xml,
	* src/tests/xml/z_max_plus_ref.xml,
	* src/tests/xml/fmp_ref.xml: Fix. Add an extraneous blank line at end 
	of file.

	* src/tests/xml/session_ref.xml
	* src/tests/xml/session_ref.dot,
	* src/tests/xml/xml_session_tester.cc: Add tests for XML sessions.
	* src/tests/xml/tester.sh: Update accordingly.
	
	* include/Makefile.am,
	* src/tests/xml/Makefile.am,
	* src/demos/xml/Makefile.am: Update.

	* doc/xml/vaucanson.xsd: Fix maxOccurs parameter for sessions (was set 
	to 1, set to unbounded).