title: Polymer iron-form validate at least one checkbox is checked
body: <p>I'm using the iron-form component with Polymer, and trying to validate if at least one checkbox in a group of checkboxes is checked. </p>
  
  <p>When I call submit() on an iron-form the validate function is called. How would I add the custom validation rules to to make sure at least one checkbox is checked?</p>