Hand over full JSON-Payload to the app

Hi,

it would be great if the plugin would not just hand over the "mesage" and "msgcnt" variable to the app, but the full JSON-object.
In my proposal, the whole JSON-object from intent.getExtras() is handed over in the "payload"-property of the root element, except for the system-properties "from" and "collapse_key", who are added to the root element. Strings, arrays and objects are supported, numbers and boolean values are handed over as strings (not the most elegant solution, to be honest). For backward compatibility, the "mesage" and "msgcnt" values are also added to the root element.