Add access-callback per public field

#180 

Next is adding tests.
> allow == ignored

That is for the final grant. Implementing access callbacks are the ones that care about the difference.

See https://github.com/Gizra/restful/issues/192



> Anyway, can this be merged? :)

Sure, let's not hold this any longer. I'll open a follow up and we can continue the discussion there.