Implement negative binomial and binomial regression in new framework

[slinderman]

Now that we've implemented Bernoulli, NB and binomial are very simple extensions. We just need to implement the proper a_func, b_func, and c_func methods to extend the base class. See https://github.com/slinderman/pypolyagamma/blob/master/pypolyagamma/distributions.py#L278 for guidance.