Requests-http-signature

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- Implemented support for (created) and (expires) special headers (14)

- Raise informative error when unable to compute body digest

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- Support using a Signature header instead of Authorization header.

- Add RSA512 algorithm.

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- HTTPSignatureAuth.verify: Rely only on request.url, not path\_url

- Docs improvements (PR 1 from kennethreitz/patch-1)

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- Require key ID

- Make verify a class method for clarity

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Initial release.