refactor hmm/models.py

[murphyk]

Our hmm/models.py is getting too big. We could just use that for BaseHMM and factor out each subclass into its own file, so we would have ar_hmm.py, gaussian_hmm.py, categorical_hmm.py, poisson_hmm etc. This is more modular since we can encapsulate model-specific logic (eg M step) into separate files.