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/walletdb.h

@@ -68,9 +68,11 @@ private:
     void operator=(const CWalletDB&);
 public:
     bool WriteName(const std::string& strAddress, const std::string& strName);
-
     bool EraseName(const std::string& strAddress);
 
+    bool WritePurpose(const std::string& strAddress, const std::string& purpose);
+    bool ErasePurpose(const std::string& strAddress);
+
     bool WriteTx(uint256 hash, const CWalletTx& wtx)
     {
         nWalletDBUpdated++;