Add Heroku Button for one-click deploys from README

Hello!

This changeset adds an `app.json` manifest that describes everything necessary to deploy this application to Heroku, as well as a [Heroku Button](https://blog.heroku.com/archives/2014/8/7/heroku-button) so developers can click to deploy a copy of this example and try it out with minimal time and effort.

If you'd like to merge this, feel free to add or remove keywords from the list in the app manifest (so far, I've added "live-streaming", "broadcast" and "cdn").