Add Bitcoin block hash to proof-of-freshness

Rational as Joanna Rutkowska put it:

1. Simple
2. Doesn't trust external organizations
3. Doesn't require working RSS feeds to be verified

Blockchain.info's API is used, with the 10th most-recent block hash used rather
than the most recent to (drastically) reduce the risk of a re-org invalidating
the hash used. That said, if that does happen (or blockchain.info screws up)
it's not the end of the world: we still have the RSS feeds as backup.

utils/proof_of_freshness_generator.sh

@@ -14,3 +14,4 @@ feedstail -1 -n5 -f {title} -u http://feeds.bbci.co.uk/news/world/rss.xml
 
 feedstail -1 -n5 -f {title} -u http://feeds.reuters.com/reuters/worldnews
 
+curl -s http://blockchain.info/blocks/?format=json | python3 -c "import sys, json; print(json.load(sys.stdin)['blocks'][10]['hash'])"