[noise] Revision 31 draft

[Trevor Perrin]

I'd like to merge this in a day or so, so people see the new notation,
so would appreciate any proofreading.

I also improved the citations/references handling:

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

Changes:
 - New pattern notation
 - Remove "secondary symmetric key" feature
 - Added proper BibTex/Pandoc references


Trevor