FeinCMSModelAdmin breaks when using a custom model manager

`FeinCMSModelAdmin` does not use first manager in model like Django's admin does, which might conflict with the `objects` manager.

I'm using just the `FeinCMSModelAdmin` of FeinCMS for a MPTT model. It breaks because the manager defined as `objects` filters some items, but the first manager (which the admin uses) shows all objects. FeinCMSModelAdmin should use the first manager to not have some conflict.