Support callbacks when jobs finish

[larskotthoff]

Allow the user to specify a callback for when one/all jobs are finished. Should probably be specified when starting the workers, i.e.

$start_workers(fun, n, callback_all_done, callback_one_done)

Callback functions could be given the returned results for the job(s). This would allow to monitor progress easily and run e.g. analysis of results without synchronously blocking.