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.cpp

@@ -54,12 +54,12 @@ void WalletStack::removeAllWallets()
     mapWalletViews.clear();
 }
 
-bool WalletStack::handleURI(const QString &uri)
+bool WalletStack::handlePaymentRequest(const SendCoinsRecipient &recipient)
 {
     WalletView *walletView = (WalletView*)currentWidget();
     if (!walletView) return false;
 
-    return walletView->handleURI(uri);
+    return walletView->handlePaymentRequest(recipient);
 }
 
 void WalletStack::showOutOfSyncWarning(bool fShow)