Ported support for validating against a property with the length validator to branch validator-classes

Same as previously, but ported over to the validator-classes branch.

I'm also a little bit uncertain about how the validator should react if the validation value specified is neither a number nor another (defined) property on the model. At the moment I've reused the functionality from the numericality validation – run reject without adding any error messages. 

Should close #29.