[noise] Rev30 branch

[Trevor Perrin]

I'm working on a Noise spec revision in the "rev30" branch:

https://github.com/noiseprotocol/noise_spec/compare/master...rev30
https://github.com/noiseprotocol/noise_spec/blob/rev30/output/noise.pdf

No substantive changes.

Some minor things:
 -Added reference for AEAD concept (Rogaway)
 - Wording fixes
 - Clarified a couple Security Considerations
 - Added a couple Rationales
 - Expanded Acknowledgements, added links to related work

The main additions are in 9.4, where it:
 - generalizes Noise Pipes to allow combining different patterns
besides XX/IK (e.g. NX/NK, IX/IK, XX/XK).
 - discusses "semi-static" public keys, which could be sent in the
initial session and used for the abbreviated handshake instead of the
original static public key.

I'd like to add another bullet for PSK-based resumption, but haven't
gotten to it yet.

This is a work-in-progress, I'm not going to merge it immediately, but
just wanted to give people a head's up and get feedback...

Trevor