Support for private field validation.

[behroozkhorashadi]

Currently Rave can only validate fields in a model that are exposed through public/protected getters and setters. This leaves private and some protected member fields of the model unvalidated and vulnerable to errors. We should add the ability for Rave to reflectively check private member fields and have an auto-value extension that generates validation for private and protected fields.

Alternatively we could use byte code injection to injected validation code right into the model.