Auto-detect composer installed CakePHP folder.

It feels a little bit awkward having to adjust index.php and test.php manually after baking a new repo.
And especially having to add complete lines to make composer work out of the box.

This patch makes the auto-detection work without any modifications necessary.
That's exactly the same thing, is it not? :) At least that's what I am talking about (app is basically the root dir, as well) - and tested it with [this](https://github.com/dereuromark/cakefest/tree/master-autodetect).