tvOS authorization support

– Implements Google's OAuth 2.0 for TV and Limited Input Device Applications flow, https://developers.google.com/identity/protocols/OAuth2ForDevices
– This is the same authorization flow used by the YouTube TV app (https://www.youtube.com/tv).
– The flow is based on, but not fully compatible with the IETF draft protocol specification https://tools.ietf.org/html/draft-ietf-oauth-device-flow