Added support for the CIE 1931 xyY chromaticity-luminance color space.

This adds support for conversion to and from the x-y chromaticity space, while preserving luminance information for the reverse transform (as per CIE 1931 xyY).

I have a few concerns with this pull request:

This is the first Color type that is not in block caps. However, I felt that `XYY` would produce even more ambiguity.
One of the fields within the type is case sensitive (`c.y` vs `c.Y`), but I honestly wasn't sure what else could be done.