The improvements include:
* HTMLPurifier cache is stored in localcachedir
* allowobjectembed changes are not ignored any more
* the cache keys include revision and all options which makes
this suitable for local caches on cluster nodes
* unchanged test is replaced by "true" value which should
significantly improve performance
* removal of purge_all_caches() hack for directory recreation
* comments and coding style cleanup
Petr Škoda