Updated documentation to 2.2

- Added big note about changes in 2.2 for translation
- Updated references to Zend\I18n\Translator\Translator to either
  Zend\Validator\Translator\TranslatorInterface or Zend\Mvc\I18n\Translator.
Merge pull request #864 from weierophinney/feature/validator-updates

Update Validator documentation for 2.2