Pickled Classifier Naming

Case:
./train_classifier ../some_dir/corpus
This will place the classifier as /home/user/nltk_date/classifiers/../some_dir/corpus_NaiveBayes.pickle
which is equal to /home/user/nltk_date/some_dir/corpus_NaiveBayes.pickle

I assume the purpose of NLTK Trainer is to keep all classifiers in nltk_data/classifiers so I fixed the code by regex stripping all leading folders from the corpus name.