Added support for the DIN99 UCS and color difference metric.

My first work in Julia and my first github pull request - bear with me!

I added in support for the DIN99 uniform color space. This space is used mostly in European industry, though it has some applications elsewhere. The intent of the space is to correct perceptual nonuniformities in CIELAB to allow for a euclidian difference metric.

Some simple test code is generated, though I will not have a dataset to confirm the metric (though I believe it is quite straightforward) for several weeks.