First pass at OpenSesame plug-ins

I created a set of plug-ins with the same functionality as the EyeLink plug-ins. So far, I've tested it only with the dummy trackers, and it probably won't work right away with a real tracker (for sure not with the EyeLink, because it will require some modification to the camera image routine). But the basic structure should be there. I also added an example experiment as `/examples/opensesame/pygaze_plugins.opensesame`.

Progress!