Adding missing `guest` method to `Authentificator` interface

Within `auth` filter provided with `laravel/laravel` the `guest` method of `Authentificator` interface is used (see https://github.com/laravel/laravel/blob/develop/app/Http/Filters/AuthFilter.php#L47). For now the `Guard` class implementing that interface has that method (see https://github.com/laravel/framework/blob/master/src/Illuminate/Auth/Guard.php#L115), but interface itself doesn't.

If anybody would need to implement their own authentification just based on interface then they will get Fatal Error because `guest` method would be called no matter what regardless of what interface says.