Further fixes and improvements to OpenSesame plug-ins and EyeLink functionality

Again, a bunch of fixes and improvements that I ran into while implementing an experiment using the OpenSesame plug-ins. It's starting to shape up nicely.

One thing that I ran into again, is that the way that the config options are stored, in the `defaults` and (optionally) `constants` modules, carries some disadvantages that are pretty hard to work around. This is related to #5, but I'll close that one and open a different issue to outline the issue in a bit more detail.