Notify the user when his roll has been processed

[AndreMiras]

Either using in-app notifications or directly with system notifications.

Here is a quick roadmap how I see that for now:

1. dumb roll result pulling
2. implement https://github.com/kivy/plyer notifications

Improvements could include:

• use a background service to show notifications even when the app closed
• more clever pulling or even web sockets (https://etherscan.io/apis#websocket)