Stop calling Foo.objects.create() when it doesn't break (Closes #23).

This will be properly fixed in v2.0.0; the current heuristic is:
- If the user defined a custom _create method, use it
- If he didn't, but the associated class has a objects attribute, use TheClass.objects.create(*args, **kwargs)
- Otherwise, simply call TheClass(*args, **kwargs).

Signed-off-by: Raphaël Barrois <raphael.barrois@polytechnique.org>