Run camper_van as a daemon

This is a rather crude solution but the easiest one in the way camper_van is
currently set up.

To daemonize a process run camper_van with `-d/--daemon`. A PID file (set to
/var/run/camper_van.pid by default) can be specified using `-i/--pid` (`-p` is
already taken by an SSL related option).

I haven't attached any tests since I'm not really sure on how to test a
daemonized process since said process would also run test related code (due to
`Process.daemon` forking the currently running process).