Fire GitHubPayloadEvent

When a new payload comes to the gate, an event is fired.

This event is an instance of the GitHubPayloadEvent class and
contains three properties:

• the coming door
• the GitHub event type, per https://developer.github.com/webhooks/#events
• the payload, as a standard class object, JSON decoded

Components listening to this event could rely on the fact the
message has already been validated.