roundup tests for train_classifier

At your suggestions, I added some more roundup tests for train_classifier.
I covered some of the stuff I added earlier such as correctly passing arguments to GradientBoostingClassifier, using word counts and max_feats.
In addition, I added a test for --multi --binary.  It uses the nltk problem_reports corpora. The dataset isn't actually multi-categorized (each item belongs to only 1 category), but it at least exercises the separate --multi --binary code path.
nltk_classifier.sh passes for every test except it_trains_movie_review_maxent which is failing due to a missing scipy package.  I believe my scipy is up to date, but since I didn't touch that area I'm not sure I caused the issue or if nltk_trainer has simply fallen out of date with the latest scipy