Changed XCache storage engine to check for appropriate php.ini configura...

Updates the isSupported method to retrieve xcache.admin.enable_auth from the ini settings and make sure it is set properly.  This will avoid future issues of Joomla choosing XCache as the caching engine when it is not really enabled for it.

Modified removeAll and getAll to check with isSupported before trying to call xcache_list.  This allows Joomla to save configuration if for some reason the website was mistakenly configured for xcache.[others xcache_list errors out and the file save is lost]

http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=20476&start=0