Chinese POS tagging

This includes some refactoring of
-the ChainPosTagger class
-ChainPosTrainer class

And the addition of 
-subclasses and companion objects of ChainPosTrainer for training on specific tag sets and corpora
-subclasses and companion objects of ChainPosTagger for trained models and feature extraction specific to corpora and tag sets
-a POS tag domain object for Chinese Tree Bank 7.0
-a PosTag subclass and labeled subclass for Chinese Tree Bank POS tagging

I may make some more changes to the feature extraction for the Chinese POS tagger, but the accuracy is better than 91% when trained on the first 90% of CTB7, then tested on the final 10%.