Convert a number field to a python number.

def nti(self, s):
        """
        Convert a number field to a python number.
        """
        # There are two possible encodings for a number field, see
        # itn() below.
        if s[0] != chr(0x80):
            try:
                n = int(self.nts(s) or "0", 8)
            except ValueError:
                raise ValueError("invalid tar header")
        else:
            n = 0
            for i in xrange(len(s) - 1):
                n <<= 8
                n += ord(s[i + 1])
        return n