Add a way to automatically fetch queues with new priorities during the run

Closes #2
Allow worker to run even if no queues yet

Closes #3

It will sleep for `fetch_priorities_delay` in a while loop to wait for
an available queue.
Don't ignore caught signals while waiting for a queue at start - cf #3
Stop catching signals when done

Closes #1
Workers update status callbacks are now an internal API
Bump version to 0.0.3