data versioning concept which maps package directories to their python 3 counterparts, if a mapping exists, cf https://github.com/nltk/nltk/pull/359
make nltk.downloader recognize zip subdirectories for the new data versioning format
updated nltk.data so it uses the new data package versioning format, and raises an exception when a package which should exist, and loaded by the user, does not; cf https://github.com/nltk/nltk/pull/359