Small cleanup in models.base

Since each base can only be a Model subclass it always has the same metaclass and therefore always has a _meta attribute except the Model class itself.

Push to a design decision, since this sounds about right but such a core change needs a bit of thinking.

I can also suggest the renaming of the first argument of ModelBase.__new__ from cls to metacls for clarity, because it really is a ModelBase metaclass. It can be a source of confusion in future development and for people trying to understand what's happening since it's not clear from the code that cls here is NOT a newly created Model or Model-based class.

Oh, and ModelBase can be renamed to something like _ModelMetaclass because it is not a BASE class for Model it is a CLASS of Model class. And with first underscore if it is not intended for importing or customization.

And, if we are talking about the design decision, I would also note that classmethods of Model (add_to_class and _prepare) can be safely moved to its metaclass as normal (not class) methods. It has the advantage of separating the logic of Model classes and Model instances.

Perhaps you could try to get a discussion going in the django-dev group.

fixes the previous patch

I mistakenly thought that only Model subclasses can be in bases. New patch fixes this.

small optimization of previous patch

even more optimisations

Link to the discussion:
http://groups.google.com/group/django-developers/browse_thread/thread/0073b756f35b2030?hl=en

filter(lambda b...) changed to list comprehension

(In [7098]) Fixed #6214 -- Added some small optimisations to model class initialisation. Thanks, Ivan Illarionov.