Autoload as phar stub

Hi,

Sorry for the delay. I finally had time to push this one line of code to fix #382.

- adds the `autoload.php` as the stub
- `rb.phar` can now be used with CLI applications without exiting

I was about to recommend [php-box](https://github.com/kherge/php-box) to build the phar but it seemed like an overkill just to get rid of a default stub.

Cheers.
the regex makes `::buildFromDirectory` do not append `loader.php` while iterating over files.