Create a new label

        Arguments:
            doc --- first document on which the label must be added (required
                    for now)

def create_label(self, label, doc=None, callback=dummy_progress_cb):
        """
        Create a new label

        Arguments:
            doc --- first document on which the label must be added (required
                    for now)
        """
        if doc:
            clone = doc.clone()  # make sure it's serializable
        r = self.index.create_label(label, doc=clone)
        return r