Make a label from a kvlayer row.

def _label_from_kvlayer(self, k, v):
        '''Make a label from a kvlayer row.'''
        (content_id1, content_id2, subtopic_id1, subtopic_id2,
         annotator_id, inverted_epoch_ticks) = k
        epoch_ticks = time_complement(inverted_epoch_ticks)

        (unpacked,) = struct.unpack('B', v[0])
        value = (unpacked & 15) - 1
        rating = (unpacked >> 4)
        meta = None
        if len(v) > 1:
            meta = cbor.loads(v[1:])

        return Label(content_id1=content_id1,
                     content_id2=content_id2,
                     annotator_id=annotator_id,
                     value=value,
                     subtopic_id1=subtopic_id1,
                     subtopic_id2=subtopic_id2,
                     epoch_ticks=epoch_ticks,
                     rating=rating,
                     meta=meta)