Got Amazon Glacier working as a full-fledged special remote.

(Well, I think it works... Since it takes 4 hours to get data out,
which is longer than the time it took me to sign up for Glacier and
write the special remote ... I've yet to fully test it!)

Thanks to Robie Basak for writing glacier-cli, and developing the intial
hook remote support. Also thanks to Peter Todd for pointing out that
Glacier cannot store empty files, which had to be worked around in the
special remote.

Of course the 4 hour delay on retreval makes Glacier interesting. For now,
you have to run "git annex get" twice, once to queue the retrieval, and a
second time in 4 hours to get the file(s). There is a helpful example in
[[tips/using_Amazon_Glacier]].

The real complication though, is that Glacier's inventories take a long
time to get, and can be out of date. So glacier-cli caches inventory info.
I didn't feel comfortable making git-annex trust that information,
so it'll refuse to trust that Glacier has a copy of a file when dropping
it. There's a `--trust-glacier` switch to override this default paranoid
behavior when dropping files.

----

Tomorrow ... er, tomorrow is Thanksgiving trip start.

Next weekend: Webapp configurator for glacier, and maybe something
to get the assistant to detect when jobs are complete and finish
retrievals from Glacier, automatically.