Distinction between weigthed and unweighted WindowDiff
Correction of bugs in the proposed version
Merge remote-tracking branch 'DavidDoukhan/master'
Merge branch 'master' of https://github.com/nltk/nltk
incorporated David Doukhan's pull request https://github.com/nltk/nltk/pull/345 , specified float return type, added doctests, misc formatting fixes