Feature: NestedValidator for validating nested resources

* ActiveModel::FormObject module includes the Validators module.
* The User model in the dummy app has validation on the name attribute.
* The UserSignup form object in the dummy app uses the NestedValidator.
* The signup view in the dummy app displays validation errors.
* Cucumber feature for invalid signup attempt to test for nested errors.
* Set config.i18n.enforce_available_locales = false in dummy app silence Rails 4 locale warnings.
Feature: FormObjects support Validation callbacks

* ActiveModel::FormObject module includes the Validations::Callbacks module
* UserSignup form object in dummy app defines a before_validation callback
Feature README docs updated

* Example of validation callbacks
* Example of NestedValidator use