Add the ZMQCert class

The ZMQCert class wraps the CZMQ zcert API.

Currently, the ZMQCert class can be created (see ZMQCert#__construct),
destroyed (see ZMQCert#__destruct), and have its public and secret keys
inspected.
Allow ZMQCerts to be tested for equality

Add the ZMQCert#equals and #__clone methods.

Also, allow the ZMQCert to have its public and secret text inspected
(see the ZMQCert#getPublicTxt and #getSecretText methods respectively).
Allow ZMQCerts to get and set metadata
Allow ZMQCerts to be saved

ZMQCert#save saves the public and secret certificates to separate files,
whereas ZMQCert#savePublic and ZMQCert#savePrivate them to the
specified files.