uv_after_work_cb backward compatibility / test case for multiple query results

This patch adds an explicit cast to the 4th argument of 'uv_queue_work' calls for backward compatibility (see https://github.com/joyent/node/wiki/Api-changes-between-v0.8-and-v0.10 and https://github.com/rbranson/node-ffi/commit/fdeff41ae8b1cca31d4707d7b61253c45181b8fa). Without this patch I was unable to compile the latest code from master on node 0.8.18.

I've also added a test case for multiple results returned by a single query (coverage for the patch I submitted to node-db-mysql - https://github.com/mariano/node-db-mysql/pull/101)