Support RSA signing

[aaronpk]

It looks like this library currently only supports HMAC signing. It would be great if it also supported RSA signing with a private key!

[liamdennehy]

PR #35 implements RSA signing

• Supports SHA256 (you really shouldn't be implementing anything new on SHA1)
• Tested against servers enforcing Signature & Authorization headers
• Does not contain unit tests, but with some pointers I am sure this can be completed.

[liamdennehy]

Update: #35 Has full suite of unit tests, and in combination with #43 now passes the Reference Tests.

[liamdennehy]

I have integrated RSA in my own project along with a number of others (Elliptic curves, DSA) that can pass the RFC reference tests, and published this in packagist. Documentation for the entire library is published at Read the Docs: http-signatures-php - incomplete but being expanded regularly.