Added QRCode generation functions via libqrencode. Switch on with USE_QRENCODE=1.

Amended build docs for Linux and OSX, and OSX makefile.
Added package 'qrencode' to gitian.yml

contrib/gitian-descriptors/gitian.yml

@@ -16,6 +16,7 @@ packages:
 - "libssl-dev"
 - "git-core"
 - "unzip"
+- "qrencode"
 reference_datetime: "2011-01-30 00:00:00"
 remotes:
 - "url": "https://github.com/bitcoin/bitcoin.git"