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.
Looks like `Hello`, `Ack` `{Un}Register`, `Ping`, and `Purge` are only called by `Run()`...removing those from the interface would make for a slimmer `NoWorker` implementation.