Add noncentral hypergeometric distribution

There are two noncentral hypergeometric distributions (Fisher and
Wallenius). Fisher's distribution is used for tests based on contigency
tables as the Fisher exact test (necessary for e.g. computing confidence
intervals).
Add Wallenius' noncentral hypergeometric distribution
Merge pull request #255 from BigCrunsh/add_noncentral_hypergeo

Add noncentral hypergeometric distribution
WIP extend hypergeometric distribution

- add support
- add mode, skewness, kurtosis
- add tests