Mandelbrot set demo

I used vispy to make a cool little Mandelbrot set demo. It runs very smooth on my laptop's Intel integrated graphics, and I tested it on Python 2 and 3. The ``min_scale`` and ``iterations`` parameters`` can be tweaked to balance quality with performance. I don't know if this fits better in the demo directory or the tutorials/gloo directory. 

Let me know if it needs work!