Expose ZMQ_MAX_SOCKETS_DFLT as a constant

Expose ZMQ_MAX_SOCKETS_DFLT as ZMQ::CTXOPT_MAX_SOCKETS_DEFAULT.

Additionally, update the 032-contextopt.phpt test case so that it won't
fail if ZMQ_MAX_SOCKETS_DFLT changes.

Fixes mkoppanen/php-zmq#118
Document the ZMQCert and ZMQAuth classes
Add the ZMQCert class

The ZMQCert class wraps the CZMQ zcert API.

Instances of the ZMQCert class can:

* have their public and private text inspected, either as 32-byte binary
  strings or Z85 armoured strings
* be tested for equality
* be cloned
* store and retrieve metadata
* have their public and private test saved to and loaded from disk
* be applied to a ZMQSocket
Add the ZMQAuth class

The ZMQAuth class wraps the CZMQ zauth API.

Instances of the ZMQAuth class can:

* install a ZAP handler for a ZMQContext
* whitelist/blacklist IP addresses
* use plain or curve authentication for one or more domains
Add examples for the ZMQCert and ZMQAuth classes

grasslands.php, strawhouse.php, woodhouse.php, stonehouse.php, and
ironhouse.php are faithful translations of Pieter Hintjens' examples of
the same names, which he originally published as part of his blog post
Using ZeroMQ Security (part 2)[0].

[0] http://hintjens.com/blog:49