Fixed a bug where missing sort parsers weren't detected, and a js error ...

...was thrown

For example, tablesorter could not cope with this table:

    <table class="table table-striped table-condensed tablesorter">
      <thead>   
        <tr><th>Name</th><th>ID</th><th>Phone</th><th>Email</th></tr>
      </thead>
      <tfoot>
        <tr><td colspan="4">0 records</td></tr>
      </tfoot>
      <tbody>
        <tr><td colspan="4"><em>No results</em></td></tr>
      </tbody>
    </table>

Clicking the ID, Phone or Email headers would throw a JS 'type is undefined' error.

This pull request checks that the parser exists first.