Use autoload as phar stub

Hi,

Sorry for the delay. I finally had time to push this one line of code to fix #382.

- uses the `autoload.php` as phar stub
- `rb.phar` can now be used with CLI applications without exiting
- no more mysterious global variables

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 crazy default stub.

Cheers.