Make usage of Composer for CLI optional

There's two problems with current CLI implementation:

1 - composer `autoload.php` file is hardcoded, which means that it is making assumptions about where `doctrine/orm` has been installed, and it also makes the assumption that `doctrine/orm` is not the main package.
2 - composer is a requirement, while requiring it should just fail silently and allow the end user to use his own autoloading strategy