concrete implementation of AbstractObjectManagerInitializer for Doctrine\ORM\EntityManager

concrete implementation of AbstractObjectManagerInitializer for Doctrine\ORM\EntityManager. 

Require PR : https://github.com/doctrine/DoctrineModule/issues/168

Sample configuration : 

    return array(
        'service_manager' => array(
            'initializers' => array(
                'entitymanager_initializer' => 'DoctrineORMModule\Initializer\EntityManagerInitializer'
            ),
        ),
    );