Added support for colorspace processing

I've often found myself adding colorspace processing to my uploaders, since clients and users often don't know how to save an image for web before uploading. Almost all my image versions for web end up forcing rgb, just to be safe. IE8 does not support cmyk colorspace, and I can't remember which other browsers don't. 

Options include :rgb and :cmyk.

    process :rgb

or 

    process :cmyk

Simple as that. Rmagick and Minimagick both supported.