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/forms/addressbookpage.ui

@@ -79,6 +79,17 @@
        </property>
       </widget>
      </item>
+     <item>
+      <widget class="QPushButton" name="showQRCode">
+       <property name="text">
+        <string>Show &amp;QR Code</string>
+       </property>
+       <property name="icon">
+        <iconset resource="../bitcoin.qrc">
+         <normaloff>:/images/qrcode</normaloff>:/images/qrcode</iconset>
+       </property>
+      </widget>
+     </item>
      <item>
       <widget class="QPushButton" name="deleteButton">
        <property name="toolTip">