Support "BASE <url>" at Turtle directive

Added to Turtle in D2 change in February 2013. See:
http://www.w3.org/TR/turtle/#sec-changelog-CR-PR
Support "PREFIX <url>" as Turtle directive

    Added to Turtle in D2 change in February 2013. See:
        http://www.w3.org/TR/turtle/#sec-changelog-CR-PR
associate parsec combinators with W3C grammar rules (not completed)