corrections on the windowdiff metric

Several corrections are proposed:
1 - distinction between the weighted and unweighted variants of WindowDiff
This metric was initially proposed as being unweighted, and implemeted weighted in NLTK.
This may be confusing for those who do not know about the weighted variant
2- bug in the window size: last implementation considered window sizes of length k+1, it should be k
3- final division, proposed in all variants, but not present in the current implementation