test_listener_with_event and test_talker_with_callbacks

This is a pair of example programs to go with this pull request in rosgo_core: https://github.com/akio/rosgo_core/pull/6

Have you considered putting all 6 rosgo repositories together into one larger repository?  It feels like it would be more convenient to manage that way.  If you like the idea, I can do the work myself and submit it as a pull request to the "rosgo" repository.