Translation formset

This is in the same spirit as #131.

This creates a formset of all the translations of a model, so they can be displayed and edited in one go (using a js-based tab view for instance). It is smarter than an regular `ModelForm` set up to work on the translations model:  it uses `combine()` to trigger checks on the translatable model. This allows custom `validate_unique()` and `save()` methods to be properly called on the shared model, as well as signals being sent.

As a convenience, it also adds `language_code` to the supplied form if missing, so one only has to write the field he is interested into.

```python
# Basic use
MyTranslationsFormset = translationformset_factory(Post, extra=1)

# With custom form
class MyPostTranslationForm(ModelForm):
    class Meta:
        fields = ['title', 'excerpt', 'content', 'slug']
MyPostTranslationFormSet = translationformset_factory(Post, form=MyPostTranslationForm, extra=1)
```

The code looks big, but it is actually ~30% code, 50% testcase and 20% documentation ;)