Incorporate statistics2 implementations into Distribution

[agarie]

The gem statistics2 appears to not be maintained anymore, according to its latest version (from 2010) and the latest commit in GitHub (from 2012).

Instead of allowing a user to use it (which is essentially Ruby with some C extensions), it seems better to substitute Distribution's Ruby implementations with them, thus allowing us to maintain the code (and maybe get some extra performance from the C code).

Also, statistics2 uses the same license as Ruby.