Encrypted sockets

[ajkeller34]

It'd be nice to enable encryption functionality for sockets so that I could use ZeroMQ over a public network. It seems like it would be a pretty straightforward PR to enable setting socket options for zmq_curve, except that it appears these socket options were introduced in ZMQ v4, and ZMQ.jl also supports ZMQ v3. If a PR is welcome, would the recommended approach be to do something like this but also check the ZeroMQ version in the new definitions?

On a related note, it seems like version is declared as global const in __init__(), which the Julia documentation (modules section) says not to do when you precompile a module. In the same PR I could define const version = Ref{VersionNumber}() outside of __init__ and then copy the version in during __init__. I don't know if anyone is checking ZMQ.version in their packages, but they would have to then check ZMQ.version[] instead.