Can't syncdb on apps with empty models.py files (0.5.7)

In couchdbkit 0.5.4, it syncdb would sync the design docs of any app, regardless of whether the models.py file had a subclass of Document or not. In 0.5.7 if an app doesn't have any `Document`s then its _design directory doesn't get sync'd.

We find it useful to have apps that give extra views to other apps' data, without defining any documents of their own.