Provide payload 'clear' value for SNS, Webhook and Opsgenie services and 'incident_key' for SNS

[pouellet]

I originally planned to add it only for SNS, but after seeing Issue #132, I decided to apply it to Webhook and Opsgenie as well. Please let me know if I should do separate pulls requests.

This includes:

• Added a test helper for alert clear payload. It would be great if you could review that this is the actual payload provided by Librato when running in production.
• SNS, Webhook and Opsgenie now extract the 'clear' value out of the payload instead of hardcoding to 'normal'.
• SNS service now provides the 'incident_key'. To be honest, I am unsure about this one as it is completely undocumented at the exception of the PagerDuty and VictorOps integration. If the 'incident_key' value is unique for every combination of alert trigger/clear, then it would be really useful for the SNS integration. In any cases, I don't see any harm in publishing more information as part of the SNS message.

[pouellet]

Closing pull request. I think most of it is now fixed.