Add modularity for rdio playback key

- Get rdio playback key through rdio.mashery.com api, and set to the
  correct env variable through figaro.
- Visit localhost:3000 instead of 0.0.0.0:3000 for the supplied API key
  in readme
- Each domain will need own key (echonest.herokuapp.com)
Merge branch 'heroku-rdio-playback'

* heroku-rdio-playback:
  Add modularity for rdio playback key