LOW PRIORITY: FEATURE: Add data handling to SimplePush

Allow "data" to be send to actively connected clients. This does NOT store the data, so if a client isn't connected, the data WILL BE discarded.

Also: 

* add unit tests for Update handler
* refactor code to provide framework for adding unit tests for handlers
* removed some extraneous classes that were causing data duplication.
We could look at adding a separate struct with `json.RawMessage` fields and avoid decoding the entire envelope just to add a top-level field. Not worthwhile if the reflection and parsing overhead is small, though.