Dispatch Refactor

[k-mahoney]

Addressing issues in https://github.com/mapbox/dispatch/issues/37, refactor to normalize function and test structure, as well as error messages and general style. There is still some mixes use of callbacks and promises, can set future goal of converting entirety to one or the other for readability.

/cc @matiskay @zmully

[matiskay]

Looks good to me. I only added some comments to improve the readability of some conditions.

[k-mahoney]

Thanks for reviewing @matiskay @zmully - starting test deploys this morning.

[k-mahoney]

Working on fixing issue with error handling in github functionality, problem with unexpected return from getIssues if none are found. https://github.com/mapbox/dispatch/blob/dec5af4304323dcc7a57e2401c418d5ae552ade2/lib/github.js#L28-L40

[k-mahoney]

@zmully I think I'm good to merge, pending any corrections you want to make to the log schema here. Currently we are not logging on every successful dispatch-incoming run, but we do log on a successful GitHub issue ticket closure or PagerDuty escalation in dispatch-triage.