Add support for modifying pngs read in from filesystem or externally via

* read_png
* update_png_pixel

For example:

img_url = "http://upload.wikimedia.org/wikipedia/commons/thumb/4/47/PNG_transparency_demonstration_1.png/280px-PNG_transparency_demonstration_1.png"
read_png img_url

(0..150).each do |x|
  (0..150).each do |y|
    update_png_pixel img_url, x, y, {:green => 200}
  end
end

image 0,0, img_url