Add support for full-color images

[tokee]

The current internal structures are hard-coded for greyscale. Switching to RGB means a tripling of the cache storage requirements and makes it more challenging to match images to a given pixel color.

Maybe a reduced 6x6x6 color space (representable as a single byte, just as with grey scale) for matching would work? But it is not obvious how to locate nearest color, when there is no exact match.

Relevant StackOverflow: http://stackoverflow.com/questions/4057475/rounding-colour-values-to-the-nearest-of-a-small-set-of-colours