Add on_cleanup event to pipeline.

Some pipelines (i.e. ArchiveBot's) run threads that, if not terminated,
will prevent the usual touch STOP mechanism from stopping a pipeline.
Such pipelines need a way to be told "hey, the runner's about to stop,
so do whatever you need to do to prepare for shutdown".  This commit
adds such an event and makes SimpleRunner (i.e. what most Seesaw
sessions use) trigger it.

(Because this event requires runner-pipeline interaction, other Runner
subclasses will have to be sure that they trigger it.)
Merge pull request #56 from yipdw/cleanup-event

Add on_cleanup event to pipeline