Refactor and cleanup hvad.views.

We have some legacy code for views, that is seldom used, has but 2 tests, and is not documented at all. I believe not many projects would be using it, which should ease transition.

Useful reference for this topic: [Django's generic Form views](https://docs.djangoproject.com/en/1.7/topics/class-based-views/generic-editing/#model-forms)

This PR refactors the whole `hvad.views` module to make it conform to Django semantics. The most notable changes are:
- **_language()** is changed to `get_language()`. This method takes no argument and must return the language to use for the form, or `None` to use current language. Default implementation looks for `?language=` in the request uri.
- **_get_object()** is renamed to [`get_object()`](https://docs.djangoproject.com/en/1.7/ref/class-based-views/mixins-single-object/#django.views.generic.detail.SingleObjectMixin.get_object). Unchanged otherwise.
- The default lookup for object id now uses [`pk_url_kwarg`](https://docs.djangoproject.com/en/1.7/ref/class-based-views/mixins-single-object/#django.views.generic.detail.SingleObjectMixin.pk_url_kwarg), which default to `pk` instead of `object_id`.
- **filter_kwargs** is deprecated. It should be replaced with [`get_queryset()`](https://docs.djangoproject.com/en/1.7/ref/class-based-views/mixins-single-object/#django.views.generic.detail.SingleObjectMixin.get_queryset) or [`get_object()`](https://docs.djangoproject.com/en/1.7/ref/class-based-views/mixins-single-object/#django.views.generic.detail.SingleObjectMixin.get_object) depending on the specific use case.
- **context_modifiers** are deprecated. They should be replaced with [`get_context_data()`](https://docs.djangoproject.com/en/1.7/ref/class-based-views/mixins-single-object/#django.views.generic.detail.SingleObjectMixin.get_context_data).

Using any of those in a view still works for now, but triggers a warning pointing at the view and advising the developer to update it. The plan is to change from warnings to exceptions on release 1.3 and **drop the old names on release 1.5**.

As a benefit, hvad now offers fully featured, Django-compliant `TranslatableCreateView`, `TranslatableUpdateView` and `TranslatableDeleteView`, as well as `TranslatableFormMixin`, all of them being fully tested. Deprecation path is tested as well. Only thing amiss is documentation