amazon s3 backend

[zooko]

(originally I incorrectly posted this to #917)

The way to do it is to make a variant of src/allmydata/storage/server.py​ which doesn't read from local disk in its _iter_share_files(), but instead reads the files from its S3 bucket (it is an S3 client and a Tahoe-LAFS storage server). Likewise variants of storage/shares.py, storage/immutable.py​, and storage/mutable.py which write their data out to S3 instead of to their local filesystem.

Probably one should first start by abstracting out just the "does this go to local disk, S3, Rackspace Cloudfiles, etc" part from all the other functionality in those four files... :-)