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

src/qt/bitcoin.qrc

@@ -41,6 +41,7 @@
     <qresource prefix="/images">
         <file alias="about">res/images/about.png</file>
         <file alias="splash">res/images/splash2.jpg</file>
+        <file alias="qrcode">res/images/qrcode.png</file>
     </qresource>
     <qresource prefix="/movies">
         <file alias="update_spinner">res/movies/update_spinner.mng</file>