support ssh-agent and/or ~/.ssh loading

[cburroughs]

Followup from #10. java-http-signature controls the fingerprint generation, and the user supplied fingerprint is only used (if at all) for sanity checking.

This opens up the possibility to -- given a fingerprint -- have the library find the right key in ~/.ssh or an agent. This is how the node ecosystem now works.

[phillipross]

👍