Add CIE76 and CIE94 algorithms (#14)

!! NOT READY TO MERGE !!

The default diff algorithm will be CIE 94, as it provides results that are very close to that of CIE 2000 with a not-insignificant increase in performance. For the same image with different diff algorithms, CIE 76 and CIE 94 both resulted in a run time of 40ms, while CIE 2000 resulted in a run time of 46ms for n = 1000. 

The API consumer can choose between CIE 76, 94, and 2000 by passing a value for the optional `accuracy` parameter to `dominantColorsInImage()`. 

This is pending on #13 being merged.