Poisson naive Bayes classifier (PoissonNB)

Added PoissonNB class to naive_bayes.py to implement a Poisson naieve Bayes classifier.  Poisson classifiers have important applications in systems neuroscience.  See: Ma et Al. *Bayesian inference with probabilistic population codes*  Nat. Neuroscience 9:1432 (2006).

Much of the ``fit`` and ``_joint_log_likelihood`` methods won't work with sparse matrices. Currently, either will throw an error at ``check_X_y``.  Unfortunately, array-like and sparse-matrix overload multiplication differently, so adding sparse matrix support will require detecting what was passed to ``_joint_log_likelihood`` and branching on it or adding a new function similar to ``safe_sparse_dot`` but with the array-like (element-wise) multiplication behavior to extmath.py.

I'm happy to do either, or hold off on sparse matrix support for the time being.  Let me know which of those three you prefer.