Fix: Removed all references to Model.objects, replaced by _default_manager

In most places, hvad uses model.\_default_manager to reference the model's default manager, which is the right way to do it. I found a couple of places that have model.objects lingering around. This will change them for greater good.

I stumbled upon that while trying to switch objects to translationaware for one of my models:
```python
unaware = TranslationManager()
objects = get_translation_aware_manager(Model)
```
That made the metaclass complain. Now it works!