Refactor translation options to be able to keep track of translation fields inherited from concrete supermodels and find fields inherited indirectly (through unregistered parents). Ensure that a base is not registered after or unregistered before submodel (that would necessitate repatching the submodel).

This changes how _registry looks, now info about all seen models is stored (including unregistered ascendants of registered models), to avoid recomputing parents options when determining translation fields of a submodel. ``TranslationOption`` instances have just two dictionaries of fields: ``local_fields`` (similar to the former ``localized_fieldnames``) and ``fields`` to match Django naming and precompute only what's necessary.
Separate registry for manager shouldn't be needed any longer -- the main registry caches options for all seen models.
Now it might be possible to optimize the manager by precomputing or caching ``fields_to_translatable_models`` on translation options (instances).
Adapt management commands and a debug message (by using ``get_registered_models`` rather than directly ``registry``).
Adapt admin to new translation options, allowing it to manage translation fields inherited from concrete supermodels.
``TranslationOptions.localized_fieldnames_rev`` is no longer available, but should be replaceable by field.translated_field (may be readded as a ``TranslationOptions`` method if it's useful for something).
Test registration and translation fields inheritance with a more complex model hierarchy (inspired by Mezzanine).
Small cleanup in ``FieldsAggregationMetaClass``. The manual inheritance syntax is left mostly for backwards compatibility.
Flake8, unused variable and import.
Merge pull request #137 from wrwrwr/feature/translation-fields-inheritance

Translation fields inheritance