Add logging to the popit-daemon-debian init script

This will hopefully give a bit more of an insight into why the process
sometimes restarts. The solution was adapted from this SO answer [1].

If this solution works then it would be good to make all logs go to
stdout/stderr rather than try to write then to a file, then add
the file we write to into logrotate.

[1] http://stackoverflow.com/a/12662904