Support for PagerDuty incidents

Add support for parsing PagerDuty incidents via https://github.com/github/pagerduty-incident-webhooks. Stores annotation-style metrics for recognized alerts received by PagerDuty (currently Nagios and Pingdom). Useful for tracking the volume of alerts (see http://obfuscurity.com/2012/04/Unhelpful-Graphite-Tip-1 for examples).