Add deltaE functions and lab2lch color conversion utils

Adds four color distance metrics CIE76, CIEDE94, CIEDE2000, and CMC.  Docs and tests included.  The deltaE functions should work so long as the two color arrays are conformable.  ie deltaE(x, y) works for x.shape == (3,); y.shape = (10, 10, 3) as well as when they have the same shape.

It also adds lab2lch (lch2lab) color conversion functions.  These were needed for some of the deltaE functions, but are not actually called internally.