Maintain state between runs

[scotthellman]

Alarmageddon is currently stateless. This simplifies the implementation but means that it can spam hipchat/pagerduty/email endlessly. Now is a good time to revisit this decision: With the addition of meaningful __repr__s (959a460) to allow for unique PagerDuty incident IDs, we've now taken care of a portion of what makes keeping track of state difficult.