Colorspace conversion functions.

Trying to bring our color support in line with other major graphic packages.

New methods on Color4 to convert between linear RGB and:
sRGB
Hue, Saturation, Lightness (HSL)
Hue, Saturation, Value (HSV)
CIE 1931 XYZ
YCbCr (which pretty much everyone seems to just call YUV, I'm not sure if we should follow suit here or rename the methods from YUV to YCbCr)
Hue, Chroma, Luminosity (HCY) (Y meaning luminance is very common, see YUV) 
According to the .Net naming conventions[1], acronyms with >2 letters should be not be all-caps. This should be named `FromSrgb` to follow the .Net naming conventions and match the tokens in `OpenTK.Graphics`.

[1] http://msdn.microsoft.com/en-us/library/ms229043%28v=vs.110%29.aspx