Fix the logic that picks up on the parameter type for Image#draw_contours
Added Color#zeros?
Fixed the color space constants.
Specced Color#zeros?
Made Image.new more flexible and added Image#threshold.

Now you can specify the dimensions, channels and depth when creating an
image.
Cleanup for inconsistencies and possible memory leaks.

Also simplified Image#threshold (only takes numbers as parameters now)
and other methods I had put into their own functions.