Add support for external class validators

[ejsmith]

Adds support for defining IValidator<T> classes to provide validation for class types outside of the model class where you are unable to alter the target model or would prefer to keep the model classes simple and not have the IValidatableObject interface implementation.

[niemyjski]

@DamianEdwards would this be something you would consider merging. We found this much easier to convert our existing fluent validation while leaving our models clean.