Minor bug corrections.
Improve HTTP content-type handling.
Overhaul: don't read content into memory unnecessarily, handle HTTP If-Modified-Since.
extract entries in order.

PHP's foreach goes by order the keys were added, not in numerical order.
Improve Last-Modified handling.
If php error written before headers, serve log instead of Cachable.
Update loglabel.
Class for handling Cache-Control HTTP header values.
Overhaul: add meta file & base cache age on its mtime.