Proposal: Final Definition of WebID

I have proposed a final definition of WebID based on the latest new draft

https://github.com/w3c/WebID/issues/72

Repeated for convenience:

The current new draft <https://w3c.github.io/WebID/spec/identity/index.html>
has:

*WebID*: An identifier in the form of an HTTP URI that unambiguously names
an Agent and, when dereferenced, always leads to a WebID Profile Document
which itself indicates that it describes the Agent named by the WebID.

*WebID Profile Document*: An RDF document that describes an Agent.

*Agent*: An entity that performs or can perform an action, e.g., a person,
an organization, a device.