Forks Go's reader to include patch for transparent frames

This PR forks the Go's reader code and applies the patch from https://codereview.appspot.com/13829043/.

I also included an example for resizing animated gifs, which still leads to a corrupted image.  Maybe you have some ideas.  My current guess is that the as a result of the RGBA resample the colors are changed and don't match the original palette.  If that's the case, what's the best way to re-build the gif to make sure each frame uses the same palette?