Send the scope class as part of the token
Send the scope class as part of the token

As it is right now, it is not possible to authenticate plugin requests
using `docker-auth`. The docker distribution registry expects[0] a scope
with `Type` repository and `docker-auth` is sending `repository(plugin)`
as that's what it receives in the request.

Instead of sending `repository(plugin)` we should be sending
`repository` and setting the scope class to `plugin`. The default class
is `image`, which is why this works fine right now.

Fixes #269
cesanta/docker_auth