Add luminance calculations for Recs. 709 and 601

Implement calculations of [luminance] for standard definition (perceived 
luminance) and high definition (objective luminance) based on [ITU-R] 
standards [Rec. 601] and [Rec. 709], respectively.

* Introduce Chromatist::Color class to represent a color in the RGB
 color space and perform maths on it.

[luminance]: http://en.wikipedia.org/wiki/Luminance
[ITU-R]: http://en.wikipedia.org/wiki/ITU-R "ITU Radiocommunication Sector"
[Rec. 601]: http://en.wikipedia.org/wiki/CCIR_601
[Rec. 709]: http://en.wikipedia.org/wiki/Rec._709