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.
Merge pull request #3953 from rowanj/libzmq.podspec

Add libzmq.podspec