Additional MVC Translator BC fixes

- Made MVC translator a proxy (added `__call` method)
- Added a `getTranslator()` method to the MVC translator
- Inject the `Zend\I18n\Translator\TranslatorInterface` service into the service manager if the MVC translator factory creates a new i18n Translator instance.

This PR also adds unit tests for both the MVC translator and its associated factory.