Sign the document with a third party signatory.

        :param str xml_doc: Document self signed in plain xml
        :returns answer: Answer is given from the signatory
            itself if connected.

def sign(self, xml_doc):
        """Sign the document with a third party signatory.

        :param str xml_doc: Document self signed in plain xml
        :returns answer: Answer is given from the signatory
            itself if connected.
        """
        try:
            self.client = Client(self.url)
        except ValueError as e:
            self.message = e.message
        except URLError:
            self.message = 'The url you provided: ' + \
                '%s could not be reached' % self.url