Compatibility with Referer Tools module

I have created a version of the Referer Tools module for Drupal 6, but have not released it yet. I'm filing this issue here just to show the code that was necessary to make Referer Tools compatible with Authcache. You don't have to commit this; I just wanted the code to be publicly available in case anyone else wants to get Authcache working with Referer Tools in the future. It also might be useful for reference if anyone wants to see how to get Authcache to work with other modules that do things to Drupal pages making them uncacheable.

The patch may not apply cleanly; the hunk that fails can just be added to the end of api/authcache.inc manually.