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.