Added 'stalking_notify' logic for both Host and Service based objects

[Rfferrao87]

Implements #605 when both stalking_options and stalking_notify are enabled in either host and/or service definitions; its functionality is similar to the is_volatile directive, except with a more granular approach, as it only emits notifications for the states covered by stalking_options and also only if the check output changes.

[aaronagios]

@ericloyd what do you think here?