Add Yt::Errors::MissingAuth

Distinguish between a request where the user provided no valid way
of authentication (MissingAuth) from a request where the user provided
a way of authentication that did not work (Unauthorized)
Add OAuth for Devices

It is now possible to use yt also for apps that authorize access to
YouTube through a device. The first request to authorize will fetch
a device code and a user token; once the user confirms the token in
a browser, the account is authorized.
Illustrate OAuth for Device in bin/yt

You can now run `bundle exec bin/yt` (or simply `yt` if you install
the gem) to get information about your account, channel, videos and
playlists, after you authorize the app with a device code.
Merge branch 'device'