Separated TranslatableModel into TranslatableModel stub and Translatable...

``TranslatableModel`` inherits directly from Django ``models.Model``. This under normal circumstances is but, but sometimes one might have a use case, where he must add “translatability” to an existing abstract model.

Here one is in conflict because he can not change the abstract base class. In these cases it useful to have the interface separated from the abstract base model. This PR moves the interface into a mixin class ``TranslatableMixin ``, and adds a stub ``TranslatableModel ``, which offers the well known interface.

Now this ``TranslatableMixin `` can be added to an existing abstract class derived from ``models.Model``. The class ``TranslatableModel`` behaves as it always did.

I believe, this might also help to solve issue #2.