handle_wrap: expose an `isRefed()` check to JS

This allows third-party tools to check whether or not a handle that
can be unreferenced is unreferenced at a particular time.
Notably, this should be helpful for inspection via AsyncWrap.

Also, this is useful even to node's internals, particularly timers.

Refs: https://github.com/nodejs/node/pull/5828
Refs: https://github.com/nodejs/node/pull/5827
PR-URL: https://github.com/nodejs/node/pull/5834


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>