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

doc/zmq.md

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+# Block and Transaction Broadcasting With ZeroMQ
+
+[ZeroMQ](http://zeromq.org/) is a lightweight wrapper around TCP
+connections, inter-process communications, and shared-memory,
+providing various message-oriented semantics such as publish/subcribe,
+request/reply, and push/pull.
+
+The Bitcoin Core daemon can be configured to act as a trusted "border
+router", implementing the bitcoin wire protocol and relay, making
+consensus decisions, maintaining the local blockchain database,
+broadcasting locally generated transactions into the network, and
+providing a queryable RPC interface to interact on a polled basis for
+requesting blockchain related data. However, there exists only a
+limited service to notify external software of events like the arrival
+of new blocks or transactions.
+
+The ZeroMQ facility implements a notification interface through a
+set of specific notifiers. Currently there are notifiers that publish
+blocks and transactions. This read-only facility requires only the
+connection of a corresponding ZeroMQ subscriber port in receiving 
+software; it is not authenticated nor is there any two-way protocol
+involvement. Therefore, subscribers should validate the received data
+since it may be out of date, incomplete or even invalid.
+
+ZeroMQ sockets are self-connecting and self-healing; that is, connects
+made between two endpoints will be automatically restored after an
+outage, and either end may be freely started or stopped in any order.
+
+Because ZeroMQ is message oriented, subscribers receive transactions
+and blocks all-at-once and do not need to implement any sort of
+buffering or reassembly.
+
+## Prerequisites
+
+The ZeroMQ feature in Bitcoin Core uses only a very small part of the
+ZeroMQ C API, and is thus compatible with any version of ZeroMQ
+from 2.1 onward, including all versions in the 3.x and 4.x release
+series. Typically, it is packaged by distributions as something like
+*libzmq-dev*.
+
+The C++ wrapper for ZeroMQ is *not* needed.
+
+## Enabling
+
+By default, the ZeroMQ port functionality is enabled. Two steps are
+required to enable--compiling in the ZeroMQ code, and configuring
+runtime operation on the command-line or configuration file.
+
+    $ ./configure --enable-zmq (other options)
+
+This will produce a binary that is capable of providing the ZeroMQ
+facility, but will not do so until also configured properly.
+
+## Usage
+
+Currently, the following notifications are supported:
+
+    -zmqpubhashtx=address
+    -zmqpubhashblock=address
+    -zmqpubrawblock=address
+    -zmqpubrawtx=address
+
+The socket type is PUB and the address must be a valid ZeroMQ
+socket address. The same address can be used in more than one notification.
+
+For instance:
+
+    $ bitcoind -zmqpubhashtx=tcp://127.0.0.1:28332 -zmqpubrawtx=ipc:///tmp/bitcoind.tx.raw
+
+Each PUB notification has a topic and body, where the header
+corresponds to the notification type. For instance, for the notification
+`-zmqpubhashtx` the topic is `hashtx` (no null terminator) and the body is the
+hexadecimal transaction hash (32 bytes).
+
+These options can also be provided in bitcoin.conf.
+
+ZeroMQ endpoint specifiers for TCP (and others) are documented in the
+[ZeroMQ API](http://api.zeromq.org).
+
+Client side, then, the ZeroMQ subscriber socket must have the
+ZMQ_SUBSCRIBE option set to one or either of these prefixes (for instance, just `hash`); without
+doing so will result in no messages arriving. Please see `contrib/zmq/zmq_sub.py`
+for a working example.
+
+## Remarks
+
+From the perspective of bitcoind, the ZeroMQ socket is write-only; PUB
+sockets don't even have a read function. Thus, there is no state
+introduced into bitcoind directly. Furthermore, no information is
+broadcast that wasn't already received from the public P2P network.
+
+No authentication or authorization is done on connecting clients; it
+is assumed that the ZeroMQ port is exposed only to trusted entities,
+using other means such as firewalling.
+
+Note that when the block chain tip changes, a reorganisation may occur and just
+the tip will be notified. It is up to the subscriber to retrieve the chain
+from the last known block to the new tip.