abstract factory for configs reading keys from merged config

this abstract factory can be activated in your service manager config like this:
```php
'service_manager' => array(
    'abstract_factories' => array(
        'Zend\Config\AbstractConfigFactory'
    )
);
```
it will factor a \Zend\Config\Config object for requested names like 
```php
$serviceLocator->get('MyModule\Sub\Config');
```
where in this case the key `MyModule\Sub` has to exist in the merged Config.