Batch wq log

Fixes #295.

batch_job_wq now logs using wq_specify_log.

There is a small change to WQ api. wq_specify_log now returns 0 or 1, to indicate whether there was an error opening the log file. Before, wq_specify_log did not return anything, thus this change to the API should have little impact.

@dpandiar @dthain