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/bitcoinamountfield.cpp

@@ -130,6 +130,11 @@ void BitcoinAmountField::setValue(qint64 value)
     setText(BitcoinUnits::format(currentUnit, value));
 }
 
+void BitcoinAmountField::setReadOnly(bool fReadeOnly)
+{
+    // TODO ...
+}
+
 void BitcoinAmountField::unitChanged(int idx)
 {
     // Use description tooltip for current unit for the combobox