GSSAPI/Kerberos Security Mechanism

Adds a GSSAPI security mechanism, which introduces the ability to kerberize ZeroMQ.  As given, this will introduce a dependency on libgssapi_krb5.  It will be trivial to add a configuration option to disable this mechanism, we figured we might as well get this in front of people first.

A few sock opts have been added to get this going,

* ZMQ_GSSAPI_SERVER - similar to `CURVE_SERVER`, identifies the socket as the server
* ZMQ_GSSAPI_PRINCIPAL - principal used for this socket
* ZMQ_GSSAPI_SERVICE_PRINCIPAL 64 - if acting as client, the service principal to connect to
* ZMQ_GSSAPI_PLAINTEXT - set to disable GSSAPI encryption (encryption enabled by default)

See https://gist.github.com/cbusbey/11265987 for our "gsshouse" example.  

See our [czmq fork](https://github.com/connamara/czmq) for corresponding czmq changes (pr coming soon).

Thanks!

cc @mgatny