Changed show_image. New argument wait_forever.

Also multiply by 255 in case dtype = bool
Added method put_in_255_range
Added method rectify.
Added method to calculate the psnr.
Added method to add gaussian noise to a img.

Sigma is passed as parameter.
Changed method to_gray. Now it accepts other dtype
Changed load_img method. Now it can also convert..

... to float the result.
Made the method naive_zoom more efficient.