Changed model's on_finished_workunit handler to not automatically move 
to the next workunit.  Instead we wait for the next call to next_item.
This makes the behaviour more intuitive/consistent, because it avoids us 
moving to the next item right away when we process the last item in a 
workunit.  This was the cause of #55.
Closes #55 and closes #57.
Merge pull request #58 from drusk/on_finished_workunit

On finished workunit handling