Move validate method to new module and add specs

 * Move to ActiveModel to reduce dependencies
   * Requires fallback if active_model doesn't exist (spec_helper_model_base)
 * Require nulldb to speed-up/simplify specs
 * Explicitly require rspec as development dependency
 * Add new method to make declaring this method easier
 * Existing apps using validate_required_fields will need to add:
   include ActiveModelExtensions::Validations