adding discrete distributions and chi-squared goodness of fit test

@tmcw I'd be interested in your feedback as to whether these sorts of things could be incorporated into simple-statistics. I've added `poisson_distribution`, which will generate a distribution that's useful in modeling arrivals and queuing theory, and `chi_squared`, which tests goodness-of-fit between sample data and a hypothesized distribution such as the Poisson.

Known issues: the `chi_squared_distribution_table` is only about 33% filled in, and I need to revisit the `.reverse()` for cases when there are more than 9 classes to make sure the sort's correct.

So, incomplete, but I'd like your feedback before going any farther. Code, comments, all fair game. I'm not positive what your goal is with simple-statistics. Some of these tools I'll use myself, some of it is just getting reacquainted with things I haven't done for a long time.

I'd add functions related to the Bernoulli and Binomial distributions, finish filling out the χ2 table, incorporate your comments before closing this pull request.