Major revamp:
- Added support for color palettes
- The mandelbrot set image is now generated on the fly by a special
  MandelImg type that implements the image.Image interface
- Coloring of the mandelbrot set is now done using an iter-count
  frequency histogram. This allows coloring independent of MaxIter
- Defined a few color palettes
- Allow selection of palette from web interface
- Lock image aspect ration in web interface
- Changed default aspect ratio to 1.25
- Allow larger images (up to 10240x8192)
- Enabled client caching of images (set "Expires" to one year from
  image generation time)