Added Poisson naive Bayes classifier (PoissonNB)

Poisson classifiers have important applications in systems neuroscience.
See: Ma et Al. Nat. Neuroscience 9:1432 (2006).

Added some tests specifically for the PoissonNB class.
Implemented some of the TODOs in test_naive_bayes.py while I was in there.

Added documentation of PoissonNB to naive_bayes.rst and some details
to the docstring in naive_bayes.py.

Adding documentation for PoissonNB class.