OnionCat Repository Moved to Github

It was almost exactly 10 years ago that we started to run our OnionCat SVN repository for managing the source code.

As of today, the 9th of January 2018, the OnionCat source repository has moved to Github at https://github.com/rahra/onioncat.

This also solves the mailing list problem. To be honest, the mailing list never really worked reliably, just occasionally. All issues can now be discussed directly on Github within the issue tracker.