[WIP] [2.2] [Validator] Allow Callback validator to be defined on property level

Bug fix: no
Feature addition: yes
Backwards compatibility break: no
Symfony2 tests pass: [![Build Status](https://secure.travis-ci.org/krymen/symfony.png?branch=issue-2480)](http://travis-ci.org/krymen/symfony?branch=issue-2480)
Fixes the following tickets: #2480
Todo: -