Payment Protocol: X509-validated payment requests

Add support for a Payment Protocol to Bitcoin-Qt.

Payment messages are protocol-buffer encoded and communicated over
http(s), so this adds a dependency on the Google protocol buffer
library, and requires Qt with OpenSSL support.

src/qt/optionsmodel.h

@@ -49,6 +49,7 @@ public:
     int getDisplayUnit() { return nDisplayUnit; }
     bool getDisplayAddresses() { return bDisplayAddresses; }
     QString getLanguage() { return language; }
+    bool getProxySettings(QString& proxyIP, quint16 &proxyPort) const;
 
 private:
     int nDisplayUnit;