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.
Note to self: I should look at simplifying `PluginLoaders` in the future, or adding a test application loader...[`TestServer.load()`](https://github.com/mozilla-services/pushgo/blob/dev/src/github.com/mozilla-services/pushgo/simplepush/server_test.go#L44-124) uses a similar pattern, but it's verbose compared to this.