remove notion of service -> more procuder arguments

* oauth/oauth1/client.scm: we remove the service struct. This means we
  don't hold request/auth/access urls anymore, neither key and secret in
  a single struct. This requires a bit more typing when calling some
  functions but is also a bit more flexible right now. A part from
  testing, it easily allows changing the URLS (e.g. adding extra
  parameters).