[Soc2014] Official meta API

I'm not proposing a flag like `not_supported`, I'm proposing a meaningful semantic flag (something along the lines of `virtual`, though I haven't figured out yet if it actually correlates fully with the current use of "virtual" in the meta API).

The problem with the `is_gfk` flag is precisely that we are not "treating all fields equally", we are giving GFKs a special treatment in Django core that no other third-party field gets. The thing which is not acceptable is for a contrib app to interact with Django core via a private API which is not publicly supported for third-party fields, and that's what the `is_gfk` flag currently is.