Added shutdown handler

[SabatinoMasala]

Added a shutdown handler to catch more errors (like compile-time parse errors).

[stracker-phil]

Good addition, thanks for your contribution!

Just note that the init function is called very often, e.g. a thousand times per request. register_shutdown_function will add new shutdown handlers (instead of replacing old ones), so the process_shutdown function will be called a thousand times on every request.

Better register the shutdown function inside the constructor, which is called only once.