fixed hang bug with nltk.util.clean_html, cf https://github.com/nltk/nltk/issues/448, thanks to kmike for pointing out some issues with the original code
added new methods in data.py which should be exposed to __all__
fixed some encoding problems portuguese_en.doctest: some unicode examples in the nltk book were encoded incorrectly in the doctest file
only bs4 is needed to clean html and get the text content