explicitly cast uv_after_work_cb to uv_queue_work

Per the node 0.8 -> 0.10 migration guide:
The uv_after_work_cb signature has changed to take a second integer
argument indicating status. For backwards compatibility, explicitly cast
the 4th argument to uv_queue_work.
Merge pull request #18 from mariuz/master

explicitly cast uv_after_work_cb to uv_queue_work