Implement Gradient Hash

[abonander]

http://www.hackerfactor.com/blog/index.php?/archives/529-Kind-of-Like-That.html

• More accurate than Mean Hash
• Much faster than DCT Hash without losing accuracy

Questions:

• Ditch the choice and just use Gradient Hash?
  • Or add it to the existing choices with new enum UseHash instead of `bool?

Additional options:

• Run the gradient hash on rows and columns