Add support for lab color

[elchininet]

This is a major update of the library, it adds support for CIE L*a*b color conversion. With this change the properties of the color objects have been changed to capital letters because of the next reasons:

• For consistency. For example, if the property to get the red channel is R and the property to get the rgb representation of the color is RGB it didn‘t make sense to work with objects with lowercase properties. It is more consistent if these properties are in capital letters too. E.g. { R: 255, G: 255, B: 0, A: 0.5 }
• To make it possible to have CIE L*a*b colors with alpha. E.g. {L: 100, a: 50, b: 10, A: 0.5}

Closes: #140

[uncenter]

Looks good to me!