Refactoring Agents - accept public key only

[joepio]

See docs issue

Creating an Agent is too hard, it requires the Client to find an Invite resource, which is very different from creating a Commit. Also, the client does not know the subject in advance.

• [ ] Implement resources by signature #408, which should remove the requirement that Commits have a target subject.
• [ ] Modify HTTP Authentication headers logic, remove requirement for subject.
• [ ] Change Properties for Signer, as they can now contain public keys instead of URLs. This will break things!