title: position element on another with jQuery UI position
body: <p>Heyho again,
  I have two elements and want to position one of it relative to the second one :-)
  I found and would like to use "jquery ui .position()" for that. Here is what I got:</p>
  
  <pre><code>&lt;div id="testparent"&gt;Parent&lt;/div&gt;
  &lt;div id="testchild"&gt;Child&lt;/div&gt;
  #testparent{
      position: absolute;
     margin:100px auto auto auto;
     width:300px;
     height:120px;
     background:lime; 
  }
  #testchild{
      margin:auto;
      width:60px;
      height:90px;
      background:yellow;
  }
  $('#testchild').position({
      of: $('#testparent'),
      my: "left top",
      at: "left bottom",
      offset: "0 3"
  });
  </code></pre>
  
  <p>I want the testchild to be positioned above/under the testparent. The problem is, that I do NOT want to position it horizontally relative to the parent but just vertically!!
  Is that possible :-)??</p>