attributeSetters for ElementArrayStim and unified docstrings for color

This finishes attributeSetters for ElementArrayStim (now only SimpleImageStim left!).

Somewhat more controversial, I've simply referred to documentation for GratingStim.color/colorSpace from Window.color/colorSpace and ElementArrayStim.color/colorSpace. They are essentially the same and rather than editing all of them for the new attributeSetter structure, it's all in one place now. This does require one or  two clicks more from people reading the documentation and that's the controversial part.