[Soc2014] Official meta API

Unfortunately, _meta.fields is a legacy API. We keep it internally to Django because of performance reasons (it's a cached property) but, outside of Django, it should not be used (once get_fields() is there).
As you say, naming is confusing, so I will push in the future for a deprecation of _meta.fields . If we find a big speed decrease, we can always rename the property