Fix "required" checkboxes

When you apply the HTML5 "required" attribute to checkboxes, it requires that the user check *all* of the checkboxes instead of a single one of the group. This adds a hack so that only one has to be clicked.

This fixes issue #18.
Wouldn't this remove the ```required``` attribute if the user checks a checkbox and then unchecks it?

Also, looks like using ```required``` on checkboxes isn't supported by the standard (http://www.w3.org/html/wg/tracker/issues/111?changelog), so maybe you could go with a JS only check without setting the DOM attribute?