Support OpenStack Swift as a backend

I implemented OpenStack Swift backend.
I am glad if it is useful and acceptable.

Best regards,
  You Yamagata

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To run test_bakthat_swift.py, a Swift cluster is required.
(It may be better to prepare for mock of swiftclient)

Swift is multi server system, but you can build a clust
with a server by SAIO (Swift All in One).
http://docs.openstack.org/developer/swift/development_saio.html