Propel2-710 Propel compatibility with Symfony 2.5 components

replaced deprecated validator class with new recursive one introduced in symfony 2.5
in order to make propel2 usable with symfony 2.5

fixes #710
Imho we should check the `Validator` class existence, to BC with Symfony 2.4: if `Validator` class exists, then it's Symfony <= 2.4 and use this class. Otherwise, it Symfony 2.5 and use the new classes.