Support Amazon S3 as a file storage backend.

There's a haskell library that looks good. Not yet in Debian.

Multiple ways of using S3 are possible. Current plan is to have a S3BUCKET
backend, that is derived from Backend.File, so it caches files locally and
can transfer files between systems too, without involving S3.

get will try to get it from S3 or from a remote. A annex.s3.cost can
configure the cost of S3 vs the cost of other remotes.

add will always upload a copy to S3.

Each file in the S3 bucket is assumed to be in the annex. So unused
will show files in the bucket that nothing points to, and dropunused remove
them.

For numcopies counting, S3 will count as 1 copy (or maybe more?), so if
numcopies=2, then you don't fully trust S3 and request git-annex assure
one other copy.

drop will remove a file locally, but keep it in S3. drop --force *might*
remove it from S3. TBD.

annex.s3.bucket would configure the bucket the use. (And an env var or
something configure the password.) Although the bucket
would also be encoded in the keys. So, the configured bucket would be used
when adding new files. A system could move from one bucket to another over
time while still having legacy files in an earlier one; 
perhaps you move to Europe and want new files to be put in that region.

And git annex `migrate --backend=S3BUCKET --force` could move files
between datacenters!

Problem: Then the only way for unused to know what buckets are in use
is to see what keys point to them -- but if the last file from a bucket is
deleted, it would then not be able to say that the files in that bucket are
all unused. Need cached list of recently seen S3 buckets?

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One problem with this is what key metadata to include. Should it be like
WORM? Or like SHA1? Or just a new unique identifier for each file? It might
be worth having S3 variants of *all* the Backend.File derived backends.

More blue-sky, it might be nice to be able to union or stack together
multiple backends, so S3BUCKET+SHA1 or S3BUCKET+WORM. That would likely
be hard to get right.

Less blue-sky, if the S3 capability were added directly to Backend.File,
and bucket name was configured by annex.s3.bucket, then any existing
annexed file could be upgraded to also store on S3.

## alternate approach

The above assumes S3 should be a separate backend somehow. What if,
instead a S3 bucket is treated as a separate **remote**.

* Could "git annex add" while offline, and "git annex push --to S3" when
  online.
* No need to choose whether a file goes to S3 at add time; no need to
  migrate to move files there.
* numcopies counting Just Works
* Could have multiple S3 buckets as desired.

The bucket name could 1:1 map with its annex.uuid, so not much
configuration would be needed when cloning a repo to get it using S3 --
just configure the S3 access token(s) to use for various UUIDs.

Implementing this might not be as conceptually nice as making S3 a separate
backend. It would need some changes to the remotes code, perhaps lifting
some of it into backend-specific hooks. Then the S3 backend could be
implicitly stacked in front of a backend like WORM.