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.
It is for the developer to decide where his Kohana application is mapped. This is not the path of the application, but the URL to where it is mapped.