Remove the internal storage of tokenId and authKey in the token.

Because the operation to generate them is async, it doesn't really make sense
to have them inside the object itself.

Having one API to get them seems simpler to deal with in the future, and avoids
future possible confusion (say people trying to use these values when they
don't exist yet).
Merge pull request #38 from mozilla-services/rework-token

Remove the internal storage of tokenId and authKey in the token.