Httpsig

- Bump min version of `jsprim` dependency, to include fixes for using
http-signature with `browserify`

- Bump minimum version of `sshpk` dependency, to include fixes for
whitespace tolerance in key parsing.

- First semver release.
- 36: Ensure verifySignature does not leak useful timing information
- 42: Bring the library up to the latest version of the spec (including the
request-target changes)
- Support for ECDSA keys and signatures.
- Now uses `sshpk` for key parsing, validation and conversion.
- Fixes for 21, 47, 39 and compatibility with node 0.8

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* Removed HTTP version from request-line, per spec (breaks backwards compatability).
* Removed auto-generation of missing Date header (ensures client compatability).


http-signature (previous)
-------------------------

- Split up HMAC and Signature verification to avoid vulnerabilities where a
key intended for use with one can be validated against the other method
instead.

- Updated versions of most dependencies.
- Utility functions exported for PEM => SSH-RSA conversion.
- Improvements to tests and examples.


Changes
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