WIP Add confidence interval for Fisher exact test

Distinction between central and minlike Fisher test
---
There are mulitple ways to define a p-value for the Fisher, or more
general, for 2 x 2 table test.  In order to implement
strongly consistent confidence intervals w.r.t the corresponding test,
we need to invert the corresponding test. Unfortunately, inverting the
minlike test does not necessarily lead to an interval. Thus, the default
version should be the central p-value with the corresponding interval.

Noncentral hypergeometric distribution
---
We need Fisher's noncentral hypergeometric distribution to compute
confidence intervals. See
https://github.com/JuliaStats/Distributions.jl/pull/255.
Merge pull request #18 from BigCrunsh/ci_for_fisher_test

WIP: confidence interval for Fisher exact test