ActiveRecord::Validator should be ActiveModel::Validator

Given the push to keep everything modular and the fact that validations have been mostly extracted from active record, having this validator base class in the active record namespace seems like a mistake. It has no dependencies on active record that I can tell but validate_with from ActiveModel relies on classes conforming to this interface.

Consequently, this class should be moved into ActiveModel so that programs that don't require ActiveRecord can benefit from the interface defined by the base class.