Translation fields inheritance

Make translation fields inherited from ascendant models (abstract or concrete, even through unregistered bases). This is a lazy replacement for the ``TranslationOptions`` inheritance.

Please read the first commit message as it describes some possibly compatibility-breaking (though rather internal) changes, and take a look at ``_get_options_for_model`` method as this is where most of the new logic sits.

Here's an (unofficial) usage [example](https://github.com/wrwrwr/mezzanine/commit/adef741b963e8bba64d684c92d9e3107223dc8d4). Should remedy what's left of issue #50 and issue #136.