reversed unintended commit for Brown corpus reader
Merge pull request #469 from wooters/master

Changed frame() to accept either a frame id or frame name
fixed nltk downloader tkinter.messagebox import
Merge pull request #471 from jskda/pull_request.nltk_download_gui

fixed nltk downloader tkinter.messagebox import
warn about missing scikit-learn + __all__
Merge pull request #472 from larsmans/sklearn

warn about missing scikit-learn + __all__
Fixed doctests involving dict objects