Implement ForeignKey support (does not support reverse relations)
Add a descriptor so that ForeignKey 'field_id' attributes work.
Attempt to rewrite related_names (ref #161).
Add tests for foreign key reverse relations, and fix some issues. Still incomplete. ref deschler/django-modeltranslation#161
Finish ForeignKey reverse relation handling (ref #161).

Also enhance tests and refactor MultilingualManager rewriting of
spanned relations lookups.
Change the form of ``related_query_name`` for implicit related_names (ref #161).
Fix flake8
Merge branch 'foreign-reverse'