[WIP] Add bindings for ZeroMQ security primitives

### Add bindings for ZeroMQ security primitives

Firstly, this is a work in progress–I've yet to implement the (much simpler) `ZMQAuth` and `ZMQCurve` classes–but I wanted to send this PR early for your thoughts and, possibly, review.

This PR adds bindings for the ZeroMQ security primitives (`zcert`, `zauth`, and associated socket options), which are introduced in @hintjen's [here](http://hintjens.com/blog:48) and [here](http://hintjens.com/blog:49).

### Breaking changes

None.

### Notes

* The bindings will *only* be available if *php-zmq* is built with [CZMQ](http://czmq.zeromq.org/), which requires [libsodium](https://github.com/jedisct1/libsodium)
* CZMQ's `zconfig_save` function currently segfaults when the filename isn't writeable. I'm going to send a PR shortly