Changed to use lists instead of tuples.
This is a massive overhaul.

Changes include the following:
 * improved efficiency of gradation code (moved multiple point-processing
   functions into one location)
 * removal of the pixel-processing functions
 * converting the nested lambdas to list comprehensions (efficiency
   improvement)
 * consolidated gradation code
Added images.
Updated README to show how to do graded shades.