[Validator] Extracted message interpolation logic of ConstraintViolation and used the Translation component for that
[Form][Validator] Added BC breaks in unstable code to the CHANGELOG
[Validator] Changed validator to support pluralized messages by default

This was implemented in order to satisfy Drupal's requirements for a
singular and a plural message whenever a message is passed to their
implementation of transChoice().
[Validator] Fixed failing test
[Validator] Added ExceptionInterface, BadMethodCallException and InvalidArgumentException
[Validator] Improved the inline documentation of DefaultTranslator
[Validator] Changed DefaultTranslator::getLocale() to always return 'en'