Implementing #9 PHP 5.3 support

The biggest downside to this is that I have to do the JsonSerializable shenanigans, but truthfully I think it's a reasonable tradeoff considering the widespread use of PHP 5.3 still.
it will be true only in the `Knp\JsonSchemaBundle\Model` namespace, but since we don't have serialisation occuring anywhere else, I don't see any problem :)