- Type hint in Silex\ConstraintValidatorFactory constructor for container
- Modifed Silex\Provider\ValidatorServiceProvider to accept an optional
  'validator.validator_service_ids' parameter wich allows users to register
  custom validators.
- Added test to verify that if users registered ValidatorServiceProvider
  using optional 'validator.validator_service_ids' parameter then the new
  Silex\ConstraintValidatorFactory will be used instead of the one provided
  in  Symfony.