[Validator] Integrated the Translator in the Validator component

Bug fix: no
Feature addition: no
Backwards compatibility break: yes
Symfony2 tests pass: yes
Fixes the following tickets: #5844, #6117
Todo: -
License of the code: MIT
Documentation PR: -

This PR allows to replace the default message substitution strategy in the validator (`strtr()`) by passing an implementation of `Symfony\Component\Translation\TranslatorInterface`. The motivation for this are both #5844 and the need to replace the translation strategy in Drupal's integration of the Validator.

In the stand-alone usage of the validator, both the translator and the default translation domain can now be passed to `ValidatorBuilderInterface`:

```php
$validator = Validation::createValidatorBuilder()
    ->setTranslator(new MyTranslator())
    ->setTranslationDomain('validators')
    ->getValidator();
```

References:

* #5844
* #6117
* #6129
* [Add a validation framework to Drupal 8](http://drupal.org/node/1845546)
* [Add the symfony validator component to core despite Symfony potentially releasing BC-breaking updates after 2.3.](http://drupal.org/node/1849564)