Re-fix #137

[jihunchoi]

This PR resolves #137.
Currently, __len__ is implemented by:

    def __len__(self):
        return math.ceil((len(self.dataset[0].text) - 1) /
                         (self.batch_size * self.bptt_len))

This is insufficient, since "substraction by 1" should be performed in a batch-level.