Use javascript instead of HTTP referer

Attached is a patch that uses javascript instead of HTTP referer to see when a user has moved away from the node they were in the process of editing.
When a node is being checked out, javascript is injected in the edit node form that fires a ajax request to checkout_release() when the page is being unloaded. This works well when the person editing has moved away from the current edit page within the same site or loaded a new website. It does not rely on HTTP referers being present. Below is the diff of checkout.module with the changes.

74,78c74
<   $items[] = array('path' => 'checkout/unload/'.arg(2), 
<        'callback' => 'checkout_pageunload',
<        'callback arguments' => array(arg(2)),
<        'access' => TRUE,
<        'type' => MENU_CALLBACK);         
---
> 
130,134d126
<         else {
<           drupal_add_js(
<           "$(window).unload( function(){ $.get('".base_path()."checkout/unload/".$node->nid."');return false;} );",
<           'inline');
<         }
171a162,163
>  * Implementation of hook_exit().
>  *
175,178c168,177
< function checkout_pageunload($nid) {
<   global $user;
<   if ($user->uid) {
<     if (db_result(db_query("SELECT 1 FROM {checkout} WHERE nid = %d and uid = %d", $nid, $user->uid))) {
---
> function checkout_exit() {
>   global $user;
> 
>   if ($user->uid) {
>     $edit_types = array('edit', 'classify', 'outline');
>     $request = array_reverse(explode('/', request_uri()));
>     $referer = array_reverse(explode('/', referer_uri()));
>   
>     if (!in_array($request[0], $edit_types) && in_array($referer[0], $edit_types)) {
>       $nid = $referer[1];
209a208
>