To test it, set up an S3 bucket (in an AWS region that is not US
Standard, for read-after-write consistency), run `ipfs init`, then
edit `~/.ipfs/config` to say

      "Datastore": {
        "Type": "s3",
        "Region": "us-west-1",
        "Bucket": "mahbukkit",
        "ACL": "private"
      },

with the right values. Set `AWS_ACCESS_KEY_ID` and
`AWS_SECRET_ACCESS_KEY` in the environment and you should be able to
run `ipfs add` and `ipfs cat` and see the bucket be populated.

No automated tests exist, unfortunately. S3 is thorny to simulate.

License: MIT
Signed-off-by: Tommi Virtanen <tv@eagain.net>