Add atbash-cipher

And I am back.
I'm not a git wizard either, but I think you can just drop the old pull request and then squash the new commit together with the old one.
This link has some info on how to do that I hope: http://stackoverflow.com/questions/5667884/how-to-squash-commits-in-git-after-they-have-been-pushed

The main problem is if others have merged in a change you have done which is then squashed later on, but since this is for a pull request on a feature branch there should not be any issue.