OCSP status is checked at a regular interval, and if the OCSP status changes for any of the certificates, the change callback is executed (restarts the server, updating the OCSP staple).

Graceful restarts/reloads, refactoring

Conflicts:
	caddy/letsencrypt/letsencrypt.go
	caddy/letsencrypt/renew.go

Merged config and app packages into one called caddy. Abstracted away caddy startup functionality making it easier to embed Caddy in any Go application and use it as a library. Graceful restart (should) now ensure child starts properly. Now piping a gob bundle to child process so that the child can match up inherited listeners to server address. Much cleanup still to do.

Lots of refinement still needed and runs only on POSIX systems. Windows will not get true graceful restarts (for now), but we will opt for very, very quick forceful restarts. Also, server configs are no longer put into a map; it is critical that they stay ordered so that they can be matched with their sockets in the child process after forking.

This implementation of graceful restarts is probably not perfect, but it is a good start. Lots of details to attend to now.

letsencrypt: Simplify timing mechanism for checking renewals

Conflicts:
	config/letsencrypt/letsencrypt.go

User can specify 'tls off" in Caddyfile to force-disable automatic HTTPS configuration