Merge: AI backtrack

The first part of the artificial intelligence library.

This provides a framework for a basic backtracking algorithm.

It is used in a standard 8 queens problem (quite elegant!), but also replace the specific solver in friendz (the new solver is a little faster in fact)

Pull-Request: #642
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>