title: Allow server-side caching in `fnServerData` in DataTables
body: <p>By default Ajax calls in DataTables avoid caching by adding a bogus field onto the url, e.g. <code>http://yoursite.com/api/?_348522852542'</code>.  I would like to disable this.  It appears that the ajax functions call <code>fnServerData</code> which is defined something like this...</p>
  
  <pre><code>    "fnServerData": function ( sUrl, aoData, fnCallback, oSettings ) {
          oSettings.jqXHR = $.ajax( {
              "url":  sUrl,
              ....
              "cache": false,
              ....
              });
        };
  </code></pre>
  
  <p>I want the value for <code>"cache"</code> to be <code>true</code> rather than <code>false</code>.  Am I missing something?  Why is this hard-coded in like this?  Is the best way to do make this change to edit the source either locally or by submitting a patch? </p>