Enable annotation mapping for Validator.

Yo,

Here's another small PR for you. This one allows users to enable annotation mapping on the Symfony `Validator` instance the `Builder` from this library constructs.

This enables the Validator annotations to be applied to a target entity. As far as I can tell, this PR is fairly safe, as it defaults to existing behavior. Of course, if you enable them and don't have Doctrine installed, it will scream fairly loudly.

-Sam