Convert the given string to an L{Identifier}.

def fromString(self, inString):
        """
        Convert the given string to an L{Identifier}.
        """
        box = parseString(inString)[0]
        return Identifier(shareID=box['shareID'].decode('utf-8'),
                          localpart=box['localpart'].decode('utf-8'),
                          domain=box['domain'].decode('utf-8'))