Separated TranslatableModel into TranslatableModel stub and Translatable...

``TranslatableModel`` inherits directly from Django ``models.Model``. This under normal circumstances is fine, 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 is useful to have the interface separated from the abstract base model. This PR moves the interface into a mixin class ``TranslatableMixin ``, and re-adds ``TranslatableModel `` as a a stub class, which then offers the same interface.

Now the new class ``TranslatableMixin `` can be added to existing abstract classes derived from ``models.Model``.

I believe, this might also help to solve issue #2.