Apply population using a pre_save signal.

It can be used with ``get_or_create`` (without resaving) and, more importantly, fixture loading.
Extend population by providing two new modes: ``default`` that sets just the default translation (only if its not given; ensuring a fallback is available; alike ``update_translation_fields``) and ``required`` that further limits population by only filling default translation of non-nullable fields (minimum to keep some level of consistency).
Add a ``populate(mode)`` toggle on multilingual query set / manager and an ``auto_populate``  context manager for fixture loading.
Refactor auto-populating: move code to constructor.

Constructor is better place than pre_save signal since it
has access to all passed arguments (field names).
Also reenabled broken test.
Remove ``_populate`` keyword.

Other ways of managing population are preferred and more elegant.
Update docs.
Merge branch 'populate-modes-cleaned'