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/bitcoingui.h

@@ -15,6 +15,7 @@ class TransactionView;
 class OverviewPage;
 class AddressBookPage;
 class SendCoinsDialog;
+class SendCoinsRecipient;
 class SignVerifyMessageDialog;
 class Notificator;
 class RPCConsole;
@@ -151,7 +152,9 @@ public slots:
       @param[out] payFee            true to pay the fee, false to not pay the fee
     */
     void askFee(qint64 nFeeRequired, bool *payFee);
-    void handleURI(QString strURI);
+
+    void handlePaymentRequest(const SendCoinsRecipient& recipient);
+    void showPaymentACK(QString msg);
 
     /** Show incoming transaction notification for new transactions. */
     void incomingTransaction(const QString& date, int unit, qint64 amount, const QString& type, const QString& address);