Config class

Creating the pull request so that I can comment some lines ;)
This foreach loop is weird... I think we shouldn't reuse `$prefix` for another purpose. Here, it's used as a mixed parameter and then as a local array variable. I've heard that type changing is a cause of bad performance in PHP (that's why Facebook have it's [hhvm](https://github.com/facebook/hhvm) project).

And why add the `'\\'` in the end? I'm not sure I understand this line well.