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/qrcodedialog.h

@@ -0,0 +1,37 @@
+#ifndef QRCODEDIALOG_H
+#define QRCODEDIALOG_H
+
+#include <QDialog>
+#include <QImage>
+
+namespace Ui {
+    class QRCodeDialog;
+}
+
+class QRCodeDialog : public QDialog
+{
+    Q_OBJECT
+
+public:
+    explicit QRCodeDialog(const QString &title, const QString &address, const QString &label, bool allowReq, QWidget *parent = 0);
+    ~QRCodeDialog();
+
+private slots:
+    void on_lnReqAmount_textChanged(const QString &arg1);
+    void on_lnLabel_textChanged(const QString &arg1);
+    void on_lnMessage_textChanged(const QString &arg1);
+    void on_btnSaveAs_clicked();
+
+    void on_chkReq_toggled(bool checked);
+
+private:
+    Ui::QRCodeDialog *ui;
+    QImage myImage;
+
+    QString getURI();
+    QString address;
+
+    void genCode();
+};
+
+#endif // QRCODEDIALOG_H