Proposal for silent failure validation

As discussed on IRC, here is a quick proposal to allow validators to silently fail.

```php
$vehicle->validate('php', array(
    'source' => 'validator.php',
    'silent_fail' => true,
));
```

This proposal has just been made for xPDOOBjectVehicule, but can easily be replicated for the other vehicules if the idea/implementation suits you.