added backwards-compat ds9 module

Following on from #6, I realized that there is now another compatibility issue: users who do ``import ds9`` will have now have their code not work because it's all been moved to ``pyds9``.  This PR adds a comatibility layer such that ``import ds9`` will still work, but displays a warning that the should switch to ``pyds9`` and ``ds9`` may not work in the future.

cc @ericmandel