Added early stopping capability with multibleu.perl (extra commits removed)

Notes:
It assumed utf8 encoding meant chinese character based bleu, which might not be right for other languages.
I agree this is not ideal. But initially I thought it'd be nice to both track the standard cross entropy in the validation call back + compute bleu (occasionally) so we can compare how well we are optimizing both. Do you think we should have both in one validation call?