Erictheise poisson plus chi squared

This will merge #61 - remaining issues:

* When the poisson distribution is selected, degrees of freedom change: https://github.com/tmcw/simple-statistics/pull/61/files#diff-e45176a67b317ecbd56fb334b53e9af4R1201 - will this happen with different distribution selections?
* Is the [plain english description like 'x or greater'](https://github.com/tmcw/simple-statistics/pull/61/files#diff-e45176a67b317ecbd56fb334b53e9af4R1217) used anywhere, or tested? If not, we should remove it.

/cc @erictheise