Increase output images from 1px x 1px  to 5px x 5px

IE8 under WindowsXP has an issue where the 1x1 images output by rgbapng result in a mostly-transparent-gradient bug. This pull request is just a simple update to functions.rb to make the output images 5px x 5px. This appears to not increase the file size of output images and fixes the IE bug.