[BC Break] Fix doctrine iterators for sf2.2 & 2.3 compatibilty

Replaces the deprecated Symfony\Component\Form\Util\PropertyPath using
use Symfony\Component\PropertyAccess\PropertyAccess new method of
accessing properties.
Added PropertyAccess requirement in composer.json
PropertyAccess requirement version precision
Merge pull request #17 from lerouquin/master

[BC Break] Fix doctrine iterators for sf2.2 & 2.3 compatibilty