Get hands dirty with kafka and spotify to try out async and state.

[DanielFroehlich]

To get a better feeling for:

• async event stuff #24, #15
• persistence of state #8
  i am going to experimentally implement the spotify oauth flow (which results in persistent state, i.e. the auth and refresh tokens).

[DanielFroehlich]

spotify backend can keep access_token alive for more then 24hour during, works now.