Why do you notify "session_key" in the sendMessage event at all?

The API is clearly mixing two separate components:
- The app-to-plugin signaling and events.
- The app-to-app in-the-wire signaling.

If you mix both (as you do) you are basically building an API which is just suitable for making "PhoneRTC" to "PhoneRTC" calls. Why don't you just **properly** document the `sendMessage` API (which is NOT documented at all as stated in #111) with just the events information needed by the app developer to intercommunicate with the PhoneRTC library?

Please let the app developer choose how to signal its application/session stuff to the other peer instead of hiding such information in a "magic" `sendMessage` event that assumes the other peer is also a "PhoneRTC" endpoint running the same Cordova app.