Move Middleware and Update README

Status
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**READY**


Description
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A mild reorganization of the client code to prevent the middleware from being required when not strictly necessary, and to use a more "current practice" README format.  


Purpose
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- [x] Rename `README.rdoc` to `README.md` and update contents accordingly
- [x] Move Rack middleware into the `Rack` module and prevent backend client from requiring it


Other Notes
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As 9d1210a introduces a breaking change, the gem version is unchanged.  I expect it should be `0.10.0`, but leave that decision to you.