Rewrite of CFG generation, adds options and fixes issue #459.
Removed obsolete file generate2.py
Merge pull request #460 from heatherleaf/master

Bugfix and rewrite of CFG generation
Merge pull request #461 from larsmans/sklearn

Updates to scikit-learn wrapper in nltk.classify
support non-numeric features in scikit-learn wrapper

And greatly simplify the code in the process by
leveraging a scikit-learn DictVectorizer.