initial pagerduty notification implementation

This should cover https://assembly.com/runbook/bounties/68 .

Pygerduty/PagerDuty supports a client & client_url parameter, if you want to link back to runbooks.io from inside of PagerDuty, you can do something like: 
new_incident_key=pager.trigger_incident(service_key, description, incident_key, details, "Runbook.io", "http://runbook.io/ID123")
Ref: https://github.com/dropbox/pygerduty/blob/40f4ebad2e60d3f1765f1f227acdb1c812d0d7d7/pygerduty/__init__.py#L548