Sparse payload

[klanclos]

The initial implementation of the Payload class was more like the Message class, with defined standard attributes that were always present. Once the Payload class was expanded to handle arbitrary keyword arguments it wasn't clear why the canonical attributes were necessary in all cases; in some situations they were an active detriment, and resulted in extra JSON serialization where the extra fields had no meaning-- like an empty 'value' assignment.

This pull request eliminates the standard attributes from the Payload, instead only using arbitrary keyword arguments. The absence of the canonical attributes means that several "is None" checks had to be converted to try/except blocks, but otherwise the flow is the same.