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/sendcoinsdialog.h

@@ -2,6 +2,7 @@
 #define SENDCOINSDIALOG_H
 
 #include <QDialog>
+#include <QVariant>
 
 namespace Ui {
     class SendCoinsDialog;
@@ -31,7 +32,7 @@ public:
 
     void setAddress(const QString &address);
     void pasteEntry(const SendCoinsRecipient &rv);
-    bool handleURI(const QString &uri);
+    bool handlePaymentRequest(const SendCoinsRecipient &recipient);
 
 public slots:
     void clear();