TrueSkill implementation

TrueSkill rating implemented. Let's see if it produces sensible numbers.

The TrueSkill code is copied from https://github.com/nsp/JSkills. I don't really understand it and it would need refactoring (it's a clone from a C# code base), but I decided to not alter the code so we could merge changes more easily. If you like, you could add the full repo as sub-repo, but I'm satisfied with copy-paste ;-)