title: Tablesorter ordering dates and empty dates incorrectly
body: <p>My tablesorter table is not sorting dates and empty fields correctly.</p>
  
  <p>I would expect empty fields to be treated like zeros, and be grouped at the top/bottom. Instead I get some dates beneath them.</p>
  
  <pre><code>$('table').tablesorter({
      widthFixed: true,
      // sort on the last name in ascending order
      sortList: [
          [2, 0]
      ],
      dateFormat: "ddmmyyyy",
      emptyTo: "zero",
      sortInitialOrder: "desc",
      headers: {
          3: {
              sorter: "shortDate"
          },
          4: {
              sorter: "shortDate"
          },
          5: {
              sorter: "shortDate"
          },
          6: {
              sorter: "shortDate"
          }
      },
      textExtraction: {
          3: function (n, t, c) {
              return $(n).attr('data-date');
          },
          4: function (n, t, c) {
              return $(n).attr('data-date');
          },
          5: function (n, t, c) {
              return $(n).attr('data-date');
          }
      },
      widgets: ['zebra', 'columns']
  });
  </code></pre>
  
  <p>See the fiddle <a href="http://jsfiddle.net/5UZ7M/2/" rel="nofollow">here</a>. Sort using the 'Date of birth' column.</p>