Added meta.security object and Security documentation

[d-stahl-ericsson]

As per issue #129.

This commit adds support for using the Strong Distribution Model for ensuring the integrity of events and authenticity of event authors. This is achieved via an optional meta.security.sdm object, with two required properties: authorIdentity and encryptedDigest.

Schemas have been updated accordingly, and one example (ArtP) provided.

[ebopalm]

a) A typo in the text section:
any more than English ir secure or French is insecure

b) Challenging the statement:
Security is not a property of the language or the protocol, but of the communication channel.
Since the communication channel is only part of the entire security aspect, a alternate expression would be:
Security is not a property of the language or the protocol only, but an end-to-end responsibility.

[d-stahl-ericsson]

Typo: Ok, fixed!

Security not being a property of the protocol: I challenge your challenge :) If you agree with anything else in that text, I don't really see how you can disagree with that particular statement? In what way is security a property of the protocol itself? And which type of security? Confidentiality, integrity or availability?

[erobged]

documentation is clearly written and examples are defined in the simple.json events.
I see no reason to withhold approval.

[rogpers-ericsson]

👍