Add a histogram transformation

This transformation converts the image into a graph representation
of the color usage in the image. The base size of the histogram is
a width of 256px.

The transformation supports four parameters:

scale: Allows you to set the width of each bar in the histogram.
          Setting a large scale will allow for anti-aliasing later.
ratio: The ratio between width and height in the generated histogram - 
         defaults to the golden ratio (1.618)
red: The color to use when drawing the red part of the graph.
green: Same for green.
blue: .. and same for blue.

Add a resize/maxSize transformation to resize the resulting image
down to the wanted size if you use the scale parameter and don't want a
size multiplied with 256.