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

`index.php` is symlinked in the public folder.

`install.php` was included relatively to the index.php location,
which is not right if the application is accessed form a symlink of
index.php. The inclusion is now relative to `DOCROOT`.

This is essential when you have to hide files like `composer.json` or
`phinx.yml` containing sensitive information.