Isolate the visible part of an application inside a public folder.

`index.php` and `install.php` are specialized for bootstrapping the
application.

This is essential when you have to hide files like `composer.json` or
`phinx.yml` containing sensitive information.

Although this will wipe Minion call from `index.php`. At this point, we should have a dedicated script for it anyway in the project root.
@enov it does not make sense to load Minion from here, just like it wouldn't make sense to load PHPUnit or anything else. Minion should end at the root of the project.

I stated in the comment that we should do something about this.