Adding Payload class to manage access to webhook payload

[davidalber]

This PR adds a class -- Payload -- to manage the access to the payload contents. At this point, it looks completely superfluous since all it's doing is forcing a bunch of changes in how we index into the payload, but this is setting the stage for further work on #72, which is going to require changes in the payload behavior.

Previously we would index the payload like you normally index into a dict: payload['a']['b']['c']. In the Payload object you index as payload['a', 'b', 'c'].

A syntactically nicer alternative involves using properties (e.g., payload.a.b.c). I implemented this first, but it was quite a bit more code, although it did have other niceties. I could make it shorter, but then decided to keep it simple. For that implementation, you can see the Payload class here and it being used here, for example.

[nrc]

Looks good, thanks!