Fix typo in luminosityDifference calculation
Tweak luminosity calculation to use `getRed:green:blue:alpha` so that it can handle different colorspaces more elegantly. Also, if that fails, it now tries `getWhite:alpha` to handle grayscale correctly.
Merge pull request #8 from robertmryan/master

Fix luminosity calculation to handle more color types; fix typo in luminosityDifference calculation