A few bug fixes and enhancements for evaluating classifiers on large datasets

I found your nltk-trainer, and it worked great as a basis for comparing different classifiers for my project.  While working on it, there were a few bugs that I fixed and some pain points I had in dealing with large datasets.  I changed some code to use generators rather than lists for intermediate processing, and I refactored the code to only read in the dataset once and changed it to only score word features based on the training set rather than the test set.