Test for proper JPG CMYK reading

As discussed in #9 I have prepared a test for JPEG CMYK file. 

Image was generated with a help of http://www.rgb2cmyk.org/ site (profile setting: "No Profile" FYI) from https://github.com/jruby/image_voodoo/blob/master/test/pix.png image.

@enebo I hope you will like it. :-)