title: $.ajax(): Origin null is not allowed
body: <p>I'm just starting with <code>$.ajax()</code>. This is my code:</p>
  
  <pre><code>&lt;html&gt;
  &lt;head&gt;
      &lt;title&gt;Commons app&lt;/title&gt;
      &lt;script src="https://ajax.googleapis.com/ajax/libs/jquery/1.6.4/jquery.min.js"&gt;&lt;/script&gt;
  &lt;/head&gt;
  &lt;body&gt;
  &lt;script type='text/javascript'&gt;
      if($) console.log('jQuery loaded!\n');
  
      $(function () {
          $.ajax({
              url: 'http://en.wikipedia.org/w/api.php?action=query&amp;list=allimages&amp;ailimit=5&amp;aifrom=Albert&amp;aiprop=dimensions|mime&amp;format=jsonfm&amp;callback=?'
          })
          .done(function () { console.log('Yay!'); })
          .fail(function () { console.log('Error!'); })
          .always(function () { console.log('Complete!'); });
  });
  &lt;/script&gt;
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>For some reason I get the following error message:</p>
  
  <blockquote>
    <p>XMLHttpRequest cannot load
    <a href="http://commons.wikimedia.org/w/api.php?action=query&amp;generator=allimages&amp;gailimit=4&amp;gaifrom=T&amp;prop=imageinfo&amp;iiprop=url&amp;callback=" rel="noreferrer">http://commons.wikimedia.org/w/api.php?action=query&amp;generator=allimages&amp;gailimit=4&amp;gaifrom=T&amp;prop=imageinfo&amp;iiprop=url&amp;callback=</a>?.
    Origin null is not allowed by Access-Control-Allow-Origin.</p>
  </blockquote>