Fixed variable scoping bug.
Merge pull request #528 from jskda/master

Replace ngram functions with their iterator counterparts, cf https://github.com/nltk/nltk/issues/527
updates to NLTK URL and email addresses
Fixed bug in 'ne_chunked' demo. Some renaming of functions and tidying up.
Temporary(?) fix for zip files
Merge branch 'master' of https://github.com/nltk/nltk
return lists instead of zips for small lists, to save complicating the book
Re-implementating of FreqDist using Counter, cf https://github.com/nltk/nltk/issues/86