[MERGE] Authentication Ring spec

[Martin Pool]

Just one more thought on this: we might like to be able to store
credentials for services other than bzr branches, such as gmail (for
Keir's gsendmail code) or launchpad (for registering branches or other
rpcs), or other bug trackers.  But I think this will fit in pretty
well as they can all be mapped into urls.  We shouldn't require having
a transport for the url to be able to store it.