load metadata event listener

This PR solves https://github.com/Sylius/Sylius/issues/221 and https://github.com/Sylius/Sylius/issues/207

Every bundle which will use this listener should have the following code

```php
    if ($container->hasParameter('sylius.config.classes')) {
        $classes = array_merge($classes, $container->getParameter('sylius.config.classes'));
    }
    $container->setParameter('sylius.config.classes', $classes);
```

in BundleExtension::mapClassParameters()