Update a label

        Replace 'old_label' by 'new_label'

def update_label(self, old_label, new_label):
        """
        Update a label

        Replace 'old_label' by 'new_label'
        """
        logger.info("%s : Updating label ([%s] -> [%s])"
                    % (str(self), old_label.name, new_label.name))
        labels = self.labels
        try:
            labels.remove(old_label)
        except ValueError:
            # this document doesn't have this label
            return

        logger.info("%s : Updating label ([%s] -> [%s])"
                    % (str(self), old_label.name, new_label.name))
        labels.append(new_label)
        with self.fs.open(self.fs.join(self.path, self.LABEL_FILE), 'w') \
                as file_desc:
            for label in labels:
                file_desc.write("%s,%s\n" % (label.name,
                                             label.get_color_str()))