Is there a built-in "None of the above" checkbox option in webform? and how to create one?
This code was fine but wasn't working due to iCheck enabled and then checkboxes "aren't" cleared when the "None of the above" is checked although the counting of checked ones was updating correctly (actually the checked state IS cleared but their iCheck-added class is not and that's why their apparent checked state keeps looking as checked). I'll try find a fix to keep iCheck and make it work with maybe later but for now the following works as expected.

So for anyone that may need this.
Create a single checkbox element after the one with multi checkboxes and call it "None of the above" and give it an element custom css ( I called it 'nota' ) to use in the jQuery code to toggle checked status between the 'nota' and all other checkboxes. 

 $("input.mywfq).change(function() {
 $("input.nota").not(this).prop('checked', false);
  });

$("input.nota).on('change', function() {
$("input.mywfq").not(this).prop('checked', false);
 });


IMPORTANT: make sure iCheck is set to "none" for this element in case its enabled for the form by default.