FIX: Allow dummy mode without pylink installed

Should make it easier for people to develop Eyelink software. Only systems that actually use the Eyelink will now need to have `pylink` installed to develop eye-tracking experiments. @kdmarrett see if this solves your problem -- you might as well uninstall the `pylink` package while you're at it.

Not ready for review/merge yet, let's see how it works first.

Closes #184.