Add BubbleBabble implementation

Implements `Digest::bubblebabble` and friends. The BubbleBabble implementation is taken from the [Jacksum](http://www.jonelo.de/java/jacksum/) library, which is LGPL. As such, this PR should not be merged until licensing is signed off on by its author, jonelo@jonelo.de to ensure that the code remains compatible with JRuby's overall licensing.

If such licensing cannot be secured, then a port form the OpenSSH code should be acceptable, as the original C code is BSD-licensed.