Add get_translation() API

One of my packages (django-fluent-pages) really needs
this to avoid hacking into the `_translations_cache`.
Simplify ParlerOptions code, keep all model registrations as separate ParlerMeta object
Add proxy model tests
Simplify code, renanme parler_meta to meta everywhere.
forms: show all translated fields in the form.
Replace TypeError with ImproperlyConfigured for _validate_master()
Remove the _translations_model backwards compatibility option.
Make sure that adding a second translation table creates only one ParlerOptions object.
minor cleanups
Fix inheritance check, should only close base class, not subclasses