Made post_handler into a generic handler queue mechanism

darcs-hash:20090722140749-58103-e516ba48d032dbb2f6b7a9f54c55d51b66f6db34
Added internal core function to remove a handler

darcs-hash:20090722161510-58103-f0311a5f8fd7d049cc2bbe27b9b18c76ccdeefc0