Ported ConstraintValidatorFactory.php to Silex

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.

Usage would be:

```php
// Custom.php
public function validatedBy()
{
    return 'my.validator.custom';
}

// index.php
$app['validator.custom'] = $app->share(function ($app) {
    return new CustomValidator();
});
$app->register(new ValidatorServiceProvider(), array(
    'validator.validator_service_ids' => array(
        'my.validator.custom' => 'validator.custom'
    )
));
```