Fixing a Bleu score bug.

If any one of the p_ns is zero, that should make the entire score zero.

An attempt to resolve #789
Merge branch 'develop' of github.com:nltk/nltk into bleu_fix
bleu refactoring: removing the BLEU class.
Typo
Don’t lowercase the candidate and reference sentences.
Merge pull request #791 from dimazest/bleu_fix

Bleu score refactoring.