mb_cache: Cache page content output in files

This class can be used to cache page content output in files.

It can check if a cache file for the current page exists and whether it is still valid.

If the cache file does not exist or it is expired, the class starts capturing the current page output and stores it the cache file when the scripts exits.

If the cache file exists and has not yet expired, the class outputs the contents of the cache file and exits.