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.
Nit: Could be simplified to a value instead of a pointer; avoids explicit initialization via `new(sync.RWMutex)`.