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/addressbookpage.h

@@ -54,6 +54,7 @@ private slots:
     void on_newAddressButton_clicked();
     void on_copyToClipboard_clicked();
     void selectionChanged();
+    void on_showQRCode_clicked();
 };
 
 #endif // ADDRESSBOOKDIALOG_H