[Soc2014] Official meta API

hi @PirosB3 ! I fully agree that `get_field()` should be able to return GFKs. What I am not convinced of is that clients who are relying on GFKs being excluded should be checking specifically for GFK - I think they should be checking for a larger category that GFK fits into (even if GFK is currently the only field we know of in that category). I think this is especially important for code in Django core, which must not check for GFK specifically, since GFK is in a contrib app.