Implemented read-only remotes. This may not cover every use case around
wanting to clone a repository and use git-annex without leaking the
existence of your clone back to it, but I think it hits most of them in a
quite easy way, and allows for some potentially interesting stuff like
partitioned networks of git-annex repositories.

Zooko and I have been talking things over (for rather too long),
and I think have now agreed on a how a more advanced git-annex
Tahoe-LAFS special remote should work. This includes storing the
tahoe file-caps in the git-annex branch. So, I really need to add that
per-special-remote data storage feature I've been thinking about.
