Listener

Does this work well as a listener?
Correct me if I am wrong, but response.body is a string. We probably want to pass an object (Hash or Array) to to action_callback. If the expected response body is a JSON then this should be...

```ruby
begin
  incident_list = JSON.parse(response.body)
rescue
  fail 'Failed to parse response from PagerDuty API'
end

action_callback incident_list
```