Provide a callback function for a graceful shutdown

[fcgravalos]

First, thanks for writing this library, is very simple to use.

I think that would be useful to have a callback like the start one to be executed when SIGTERM signal is received.

daemon = Daemonize(app='my_daemon',
                           user='user', group='group',
                           pid='/tmp/daemon.pid', foreground=Fase,
                           action=start, shutdown_cb=stop, auto_close_fds=False)