net: make Server.prototype.unref() persistent

Currently, the unref() method does not remember any state
if called before the server's handle has been created. This
commit adds state to track calls to ref() and unref().

PR-URL: https://github.com/iojs/io.js/pull/897


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Brendan Ashworth <brendan.ashworth@me.com>