Fix `uv_queue_work` calls by explicitly casting uv_after_work_cb

Quoting from https://github.com/joyent/node/wiki/Api-changes-between-v0.8-and-v0.10:

>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.

This should fix #34