made login retrieval more robust

git-svn-id: svn+ssh://svn.gna.org/svn/advene/advene2@7633 3fb6f576-06d4-0310-ad11-a4b695aa9f05
spotted and documented problems with unittests

git-svn-id: svn+ssh://svn.gna.org/svn/advene/advene2@7634 3fb6f576-06d4-0310-ad11-a4b695aa9f05
fixed get_element_by_uriref consistently with its docstring

git-svn-id: svn+ssh://svn.gna.org/svn/advene/advene2@7635 3fb6f576-06d4-0310-ad11-a4b695aa9f05
added linked-data parsers and serializers

git-svn-id: svn+ssh://svn.gna.org/svn/advene/advene2@7636 3fb6f576-06d4-0310-ad11-a4b695aa9f05
made rdflib optional

git-svn-id: svn+ssh://svn.gna.org/svn/advene/advene2@7637 3fb6f576-06d4-0310-ad11-a4b695aa9f05