Support lastest stable ZeroMQ version

[OttoAllmendinger]

ZeroMQ 4.0.5 is out: https://raw.githubusercontent.com/zeromq/zeromq4-x/master/NEWS

Our current dependency check is set to ==4.0.4: https://github.com/Open-Transactions/opentxs/blob/develop/CMakeLists.txt#L182

Also the libname changed from 3.1.0 to 4.0.0, we need to allow both. I think we can just use find_library(ZMQ_LIB NAMES zmq libzmq) for non-windows platforms.

Should we just use this module? It has better Windows support than we have: https://github.com/robertmaynard/Remus/blob/master/CMake/FindZeroMQ.cmake

[toxeus]

@OttoAllmendinger we used to have just find_library(ZMQ_LIB NAMES zmq libzmq). But it would then pick the wrong lib for users that have multiple libzmq installations. Please compare 43785a0.

We learned at dotGo that if something is on GitHub it doesn't mean that's it's ready for production :) So we should probably test if it really supports Windows better. And we should also evaluate if it's worth to pull in 113 lines of code while we're doing fine with 25 which can actually be reduced, see #163.