Httpsig

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* Update to newer spec (incompatible with prior version).
* Handle `request-line` meta-header.
* Allow secret to be a PEM encoded string.
* Add test cases from spec.

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* Account for ssh now being re-merged into paramiko: either package is acceptable (but paramiko should ideally be >= 1.8.0)

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* Stop enabling `allow_agent` by default
* Stop requiring `ssh` package by default -- it is imported only when `allow_agent=True`
* Changed logic around ssh-agent: if one key is available, don't bother with any other authentication method
* Changed logic around key file usage: if decryption fails, prompt for password
* Bug fix: ssh-agent resulted in a nonsensical error if it found no correct keys (thanks, petervolpe)
* Introduce versioneer.py