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/walletframe.h

@@ -11,6 +11,7 @@
 
 class BitcoinGUI;
 class ClientModel;
+class SendCoinsRecipient;
 class WalletModel;
 class WalletStack;
 
@@ -29,7 +30,7 @@ public:
 
     void removeAllWallets();
 
-    bool handleURI(const QString &uri);
+    bool handlePaymentRequest(const SendCoinsRecipient& recipient);
 
     void showOutOfSyncWarning(bool fShow);
 
@@ -71,4 +72,4 @@ public slots:
     void setEncryptionStatus();
 };
 
-#endif // WALLETFRAME_H
+#endif // WALLETFRAME_H