Add bindings for CZMQ security primitives

### Add bindings for CZMQ security primitives

This PR adds bindings for the ZeroMQ security primitives, `zcert`, and `zauth` (exposed as `ZMQCert` and `ZMQAuth` respectively), which are introduced in @hintjens' Using ZeroMQ Security [part 1](http://hintjens.com/blog:48) and [part 2](http://hintjens.com/blog:49).

I've also converted all but the last of the examples from part 2 to PHP (see *grasslands.php*, *strawhouse.php*, *woodhouse.php*, *stonehouse.php*, and *ironhouse.php* in the *examples* directory).

### 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~~ See https://github.com/zeromq/czmq/pull/468
`zcert_public_key` returns a pointer to the associated property of the `zcert_t` struct. With that in mind, I don't think `public_key` should be freed (and `RETURN_STRINGL(..., 1)` is correct). This is also the case for `zcert_secret_key`, `zcert_public_txt`, and `zcert_secret_txt`.