Device flow description

(A)  The client requests access from the authorization server and
       includes its client identifier in the request.

  (B)  The authorization server issues a device code and an end-user
       code and provides the end-user verification URI.

  (C)  The client instructs the end user to use a user agent on another
       device and visit the provided end-user verification URI.  The
       client provides the user with the end-user code to enter in
       order to review the authorization request.

  (D)  The authorization server authenticates the end user (via the
       user agent), and prompts the user to input the user code
       provided by the device client.  The authorization server
       validates the user code provided by the user, and prompts the
       user to accept or decline the request.

  (E)  While the end user reviews the client's request (step D), the
       client repeatedly polls the authorization server to find out if
       the user completed the user authorization step.  The client
       includes the device code and its client identifier.

  (F)  The authorization server validates the device code provided by
       the client and responds with the access token if the client is
       granted access, an error if they are denied access, or an
       indication that the client should continue to poll.