Read an 8-byte data block at address (block * 8).

def read_block(self, block):
        """Read an 8-byte data block at address (block * 8).
        """
        if block < 0 or block > 255:
            raise ValueError("invalid block number")
        log.debug("read block {0}".format(block))
        cmd = "\x02" + chr(block) + 8 * chr(0) + self.uid
        return self.transceive(cmd)[1:9]