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.
Sorry, I meant adding `MaxRecords` to cap the number of `MemStorageRec` structs stored in `MemStore.Data`. `NoStore` would be a `MemStore` with `MaxRecords = 0`.