Track field name in TranslatedFields object too.
Don't apply metaclass magic or model checking to abstract classes.
Added `parler.appsettings.add_default_language_settings()` function.

This function can be used by third party packages to base their own
language-dict settings on the parler-defaults, while including extra
default options (such as "hide_untranslated").
Bump version number to 0.9.3