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/walletstack.h

@@ -20,6 +20,7 @@ class TransactionView;
 class OverviewPage;
 class AddressBookPage;
 class SendCoinsDialog;
+class SendCoinsRecipient;
 class SignVerifyMessageDialog;
 class Notificator;
 class RPCConsole;
@@ -54,7 +55,7 @@ public:
 
     void removeAllWallets();
 
-    bool handleURI(const QString &uri);
+    bool handlePaymentRequest(const SendCoinsRecipient &recipient);
 
     void showOutOfSyncWarning(bool fShow);