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/guiconstants.h

@@ -28,6 +28,9 @@ static const int TOOLTIP_WRAP_THRESHOLD = 80;
 /* Maximum allowed URI length */
 static const int MAX_URI_LENGTH = 255;
 
+/* Maximum somewhat-sane size of a payment request file */
+static const int MAX_PAYMENT_REQUEST_SIZE = 50000; // bytes
+
 /* QRCodeDialog -- size of exported QR Code image */
 #define EXPORT_IMAGE_SIZE 256