Signer.sign() now throws KeyNotFoundException
Merge pull request #21 from noaresare/pr/sign-keynotfound

Signer.sign() now throws KeyNotFoundException
createChallenge now takes request message instead of bare username

This to implement updated spec, handle non-ascii usernames and to embed
version information in the initial request.
Merge pull request #22 from noaresare/pr/request-parameter

createChallenge now takes request message instead of bare username
Moved the AgentSigner to a separate project, updated README.md
Strip padding from ASCII encoded data.
Merge pull request #23 from noaresare/pr/move-agent-signer

Moved the AgentSigner to a separate project, updated README.md