Added color constants to new namespace cv::color. 

Currently standard color names are not available in OpenCV and people have to define them each time or for each project that they work on.
Color can be represented by different types. Scalar parameters are used by most functions in the main cv namespace (e.g. cv::line), so this should be the default. Additionally, when setting the pixels of a Mat, one may want to use the Vec3b representation, so I created constants in that format, too.
The color names are based on the CSS 2.1 conventions.