cluster: add handle ref/unref stubs in rr mode

cluster: add handle ref/unref stubs in rr mode

Add ref() and unref() stub methods to the faux handle in round-robin
mode.  Fixes the following TypeError when calling `server.unref()` in
the worker:

    net.js:1521
        this._handle.unref();
                     ^
    TypeError: this._handle.unref is not a function
        at Server.unref (net.js:1521:18)

No actual reference counting is implemented.  It would effectively be
a no-op because the control channel would still keep the worker alive.

Fixes: https://github.com/nodejs/node/issues/73
PR-URL: https://github.com/nodejs/io.js/pull/2274
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>