For resource cache, check modification date of cache file.
Use sprintf() because it's funner.
Remove unused protected properties.
Rename method to simply getCacheUrl().
Code cleanup & reorg.
(finally?) restore 304 headers being sent when cache is a 'resource'.
Remove config.php until we are properly leveraging it.
Change method signatures so that $settings is optionally passed as last parameter. Then make setSettings() protected so as to match consistency with other setters.