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.

This is difficult to understand (many people might intuitively understand inverse BLEU but not know what PINC is, or why the word PINC suddenly appeared when they chose the inverse option). Is it necessary to change the ID name? If it is, you could also change it to INV_BLEU or something else that's easier to understand.