Add access-callback per public field

#180 

Next is adding tests.
I'm not sure about this.

Lets say:
callback1 - returned ALLOW
callback2 - returned DENY

So callback3 knows only about the DENY.
If we wanted we could return the array of results, but then it's still important the order of the callbacks.

That's why I prefer a simpler approach - each callback is responsible for itself. We only care out of the box about an explicit DENY.