add failing test
closed fswatchers no longer keep process alive

There was a slight race condition in `_handle` -- the method has
two async steps (realpath & stat), and before completion of either
of those steps, the FSWatch instance may have been closed. Previously,
even though the watcher was closed, new listeners would be added and
chokidar would thus keep the process alive. This behavior has been rectified.
Merge pull request #130 from chrisdickinson/close-ignores-further-events

Close ignores further events