Improve identity view helper factory

[bogutskyy]

I have a few problems with an identity view helper factory.

    public function createService(ServiceLocatorInterface $serviceLocator)
    {
        $services = $serviceLocator->getServiceLocator();
        $helper = new Identity();
        if ($services->has('Zend\Authentication\AuthenticationService')) {
            $helper->setAuthenticationService($services->get('Zend\Authentication\AuthenticationService'));
        }
        return $helper;
    }

1. First about a retrieving of auth service. Maybe will be better to use Zend\Authentication\AuthenticationServiceInterface instead of Zend\Authentication\AuthenticationService as name of the auth service?
2. By default HelperPluginManager doesn't contain main service container, and var $services can be null. But it seems in develop branch this factory class already modified.

[gszy]

1. First about a retrieving of auth service. Maybe will be better to use Zend\Authentication\AuthenticationServiceInterface instead of Zend\Authentication\AuthenticationService as name of the auth service?

See zendframework/zend-mvc-plugin-identity#9