Sending Messages HowTo/Best Practices

See conversation on gitter: https://gitter.im/jupyter-widgets/Lobby?at=60622d0814188a5edfd21f6c

Some points to be included in the tutorial:

1. Views should use `this.send` rather than `this.model.send` (https://gitter.im/jupyter-widgets/Lobby?at=6062335c56debe325c4ca07d)
2. Register the method of your choice on the python using `self.on_msg`