Added IPTColor and conversions to/from XYZ

References for IPT color space:

* Ebner, F., & Fairchild, M. D. (1998). Development and testing of a color space (IPT) with improved hue uniformity. In Color and Imaging Conference (Vol. 1998, No. 1, pp. 8-13). Society for Imaging Science and Technology.
* M.D. Fairchild and G.M. Johnson, “The iCAM framework for image appearance, image differences, and image quality,” Journal of Electronic Imaging, in press (2004). ([pdf](http://www.cis.rit.edu/research/mcsl2/icam/pub/JEIiCAM.pdf))
*  Fairchild, M. D. (2013). Color appearance models, 3rd Ed. (pp. 271-272). John Wiley & Sons.