Save the Manifest in a Private Static Variable
Turn PX::template() into a generic file parser (PX::run_file())
Limit Errors/Warnings when running without a Cache Directory

Check if the cache directory is writable before creating a manifest
file, and throw a PHP Notice if it's not.  This will allow the script
to run without a cache directory since Notices are easily ignored.
Delete old manifest files
Update cache directory get/set functions

- don't check if directory is writable when setting the cache
  directory variable

- create public function to retrieve the current cache directory