Add libzmq.podspec

Creates a CocoaPod for [ZeroMQ](http://zeromq.org)'s libzmq library.

This is the low-level C API, written in C++.  This pod is intended as a
dependency for a set of high-level bindings I'm currently building into
another pod.


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The target repo is based on the current master (to be 3.3.0, when released) with the minimal changes required to build in a CocoaPods environment (the autoconf-generated `platform.hpp`).  This file is a build product of the upstream configuration system, and so will not (ever) be merged into mainline.

Because this is based on unreleased code, it is given the **-alpha** suffix; and because the pod version may need to iterate independently to the upstream library version, it is given a trailing `1` to delineate this as the first release.