Add a first implementation of toBuffered for ColorSeparation to handle CMYK pictures.
The code has been tested with a CMYK picture, but I have not been able to test alpha picture.
The code will probably also need to be improved to handle any kind of ColorSeparation picture.
Though, it's a nice base for further developments.
Merge pull request #20 from guinotphil/jmagickconversion

Adding support for CMYK images.