Add development message provider

[onbjerg]

This PR contains a small implementation of a message provider I use in my own sandbox to test the communication between apps and the wrapper.

It is used like so:

// Message bus used to pass messages between apps and the wrapper
const bus = new Subject()

// Set up app
const app = new App(
  new Messenger(new DevMessage('app', 'wrapper', bus))
)

// Run app
wrapper.runApp(
  new DevMessage('wrapper', 'app', bus),
  '0xbitconnect'
)

Where the first parameter is the provider's own ID, the second is its target ID and the third is an RxJS subject used to pass messages between the different providers.

[sohkai]

👍

[sohkai]

Could we add the comments and example here?

[onbjerg]

yes