Call uv_unref(&g_async) to avoid keeping process from exiting.

This is a better way of fixing #47, since it requires no additional state
(e.g. g_watch_count) and reuses the existing libuv API.

I also took the liberty of removing the --forceexit option from the test
command, now that g_async no longer prevents test processes from exiting.

This reverts commit c6ddf186333a25cf52b3ab7c093a6d7b930ec829, "Call
uv_async_init lazily to avoid keeping process alive."