title: Determine if every radio group has had an option selected
body: <p>I need to make sure that all the radio groups have been answered before I enable the submit button.  If I have:</p>
  
  <pre><code>var radioBtns = $('input').filter(':radio');
  </code></pre>
  
  <p>It tells me how many radio buttons I've got, but I need to know if there are any groups that haven't had an option selected yet.</p>