RGB <-> LabSpace memoization

PR started to address issue #9 . This is work in progress for comments and direction.

** Note: This code won't yet compile.
I was thinking of memoizing the conversion functions inside the body of `dominantColorsInImage()`, that way the cached values will be discarded after processing each image, and the color values memoized will have maximum usefulness as they will be from the same image.