Add validate_unique on translated model
Clean instance translation caches after checks
Make the unique_together testable in the example app.
Different strategy for validating unique_together:

- Update the translated model fields in `_post_clean()`.
  Django also updates self.instance with form.cleaned_data at that stage.
- Override `TranslatableModel.validate_unique()` so it also checks for
  the constraints in the translated models.
add unit test for unique_together check
Merge branch 'nephila-validate_translated'
Improve runtests.py, fix sys.path for virtualenv with pth files.