Notify stream creator of stream stop

[rskew]

As a user of the Python API for creating streams in Aiko Pipelines, I need to know when a stream has stopped so I can take subsequent actions.

Currently, the start_stream method on a Pipeline enables being notified of completed frames via the queue_response and topic_response arguments. I propose also sending a message to the response queue and/or topic when the stream is stopped.

Example implementation in this draft PR #28