PINC

In this pull request, PINC is implemented by modifying the existing bleu.
PINC is an inverse BLEU and being used for measuring dissimilarity between system output and source sentence. 
To do so, we can add "inverse=true" in BLEU options.
In addition, PINC does not calculate brevity penalty, therefore we can do that by specifying "brevity=false" in BLEU options.

Some minor bug fixed in BLEU such as denominator that always be 4 in BLEU calculation and used std::abs instead of abs function in CheckAlmost function are also included.

Thanks for the commit!
This line here is actually correct.
The reason why (act != act) is being checked is to make sure that act is not NaN.
Could you please revert this to "(act != act) ||" ? Changing to std::abs is fine. Also, it might be worth adding a comment explaining the act != act part.