* patch nltk egg so that the default data location is (buildout_directory)/parts/nltk-data without explicitly specifing by NLTK_DATA environment variable.
* download collection can be overridden like 'collection = aaa bbb ccc'.

/ref http://community.slapos.org/forum/NLTK-Buildout--Download-and-install-nltk_data

/cc @klaus, @Kreisel 

/reviewed-on nexedi/slapos!58