* Finished implementing the "start" parameter to the nagios module. The backend functions already had it; this change just exposes it. This allows setting the time a Nagios outage begins rather than always starting at the time the module was run. If not provided, "start" defaults to the current time (preserving compability with existing playbooks).
* Changed default start time to None, as per comment by @goneri. This avoids initializing the variable twice.
