Adding PagerDuty Notifier, closes #567

This notifier opens an incident in PagerDuty when a backup model fails
or completes with a warning. A successful run will resolve an open
incident for that model.

Optionally, the notifier can be configured to resolve on a warning,
rather than triggering an incident. This might be wise when multiple
notifiers are in use (so warnings don't go unnoticed) and users want a
the next successful run to resolve the previously triggered incident.