Several improvements

Overview:

- Improvements to EyeLink integration
- Improvements to EyeTribe integration
- Improvements to OpenSesame plug-ins
- yamldoc docstrings in BaseEyeTracker (see http://osdoc.cogsci.nl/2.9.0/devices/pygaze/)

Since I want to release OpenSesame 2.9.0 pretty soon with PyGaze, I suggest we tag the current snapshot, perhaps with something like `pre-release/0.5.0~opensesame-1`. What do you think?