This patch does two things:

1. It prevents org-publish from perpetually republishing files
containing includes. Currently, if an included file changes, it is
published, as is the file that includes it. However, until changes are
made in the file that includes it (which may be never) its modification
time remain older than that of the included file, so it will be
republished every time org-publish is called, even if neither file has
changed. 

   - Note: This patch fixes this behavior by updating the modification
     time of the file that contains the includes. If this is deemed too
     destructive/meddlesome, we could consider alternate behaviors, such
     as setting the cached time with a simple float-time. Let me know
     what would be best.

2. It optimizes checking for included files. Currently,
org-publish-cache-file-needs-publishing visits every file in a project
to check for includes. On my underpowered box, this takes a long time
(over a minute) on a project with 1000+ files, thus defeating the
purpose of the cache. This patch causes org-publish to store information
about included files in the cache itself:

  a. If a file has changed it updates information about the files it
  includes and stores it in the cache.

  b. If a file has not changed, it checks the cache for included files
  and sees if any of those files have been updated (thus preventing the
  need to check every file in the project for includes)

Best,
Matt