Add support for toggling notifications for nagios servicegroups

Support toggling alerts by nagios servicegroups as well as hostgroups.
Also updated log messages to indicate whether alerts are being disabled
for a servicegroup or hostgroup.

Lastly updated comments to reflect that host notifications are NOT
disabled along with service notifications.
Added comments to nagios lib

Added comments to show where the "magic numbers" used in the nagios util
lib come from.
Rename missed variable

Rename variable hostgroup to servicegroup in the toggle notifications by
servicegroup function.
Merge pull request #52 from racker/dominic/support_nagios_servicegroups

Add support for toggling notifications for nagios by servicegroups