net: add net.listening boolean property over a getter

Added a listening property into net.Server.prototype indicating
if the server is listening or not for connections.

Other Server constructors that rely on net.Server should also
gain access to this property.

Also included tests for net and http subsystems.

PR-URL: https://github.com/nodejs/node/pull/4743


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>