Added a demo (with sample stereo images) for generating disparity map

I wrote a simple demo script for generating the disparity map. I had to spend some time figuring out how to do this because the blog post about BMTuner is outdated. It would be better if others don't have to go through this and they can try something quick immediately after installing stereovision package.

Feel free the change the demo images to something better.

If I find more time and this is accepted, I might add a point cloud demo as well.