add in some more bytes

def accumulate(self, buf):
        '''add in some more bytes'''
        accum = self.crc
        for b in buf:
            tmp = b ^ (accum & 0xff)
            tmp = (tmp ^ (tmp<<4)) & 0xFF
            accum = (accum>>8) ^ (tmp<<8) ^ (tmp<<3) ^ (tmp>>4)
        self.crc = accum