updated README
high level gaze contingent stuff to 'plugins' module
stablized eyetracker functionality in dummy mode
First pass at creating OpenSesame plug-ins
- Tested only with the `dummytracker` and `dumbdummy`
- Mimicks the structure of the `opensesame_eyelink_plugins`
Some minor bugfixes to the `OSMouse` class and the dummy trackers