Ported ConstraintValidatorFactory.php to Silex/Pimple

Included a modified version of `ConstraintValidatorFactory` to Silex for use with custom constraints.

Since Silex comes with `ValidatorServiceProvider` built in, I guess it makes sense to include this, in case users might want dependencies with their custom validators.

```php
$app->register(new MyNewValidatorServiceProvider());

$app->register(new Silex\Provider\ValidatorServiceProvider(), array(
    'validator.validator_factory' => new ConstraintValidatorFactory($app, array(
        'validator.my_new_validator' => $app['validator.my_new_validator']
    ))
));
```