Add ZeroMQ support. Notify blocks and transactions via ZeroMQ

Continues Johnathan Corgan's work.
Publishing multipart messages

Bugfix: Add missing zmq header includes

Bugfix: Adjust build system to link ZeroMQ code for Qt binaries

src/zmq/zmqconfig.h

@@ -0,0 +1,24 @@
+// Copyright (c) 2015 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_ZMQ_ZMQCONFIG_H
+#define BITCOIN_ZMQ_ZMQCONFIG_H
+
+#if defined(HAVE_CONFIG_H)
+#include "config/bitcoin-config.h"
+#endif
+
+#include <stdarg.h>
+#include <string>
+
+#if ENABLE_ZMQ
+#include <zmq.h>
+#endif
+
+#include "primitives/block.h"
+#include "primitives/transaction.h"
+
+void zmqError(const char *str);
+
+#endif // BITCOIN_ZMQ_ZMQCONFIG_H