Provide proof of freshness mechanism (news / blockchain based)

[emanuelb]

More information about why proof of freshness is needed:
https://groups.google.com/forum/#!topic/qubes-devel/MERCI9lEyb8 "Why proof of freshness?"

examples:
script that generate the proof for qubes canaries:
https://github.com/QubesOS/qubes-secpack/blob/master/utils/proof_of_freshness_generator.sh
froom autocanary (news based):
https://github.com/firstlookmedia/autocanary/blob/master/autocanary/headlines.py

See also "Provide blockchain based proof of freshness" issue in autocanary:
firstlookmedia/autocanary#28

fix:
add to metadata a fields that can be used to obtain information for checking 'proof of freshness' via obtaining info from news websites / public blockchains.

[ghost]

I have implemented news retrieval from the BBC World News feed in my fork:

https://github.com/thetangledweb/fugl