Add support for reading from a GifBuffer
Add support for reading from a GifBuffer

Eventually, we might want to add support for decoding via giflib
rather than calling out to gifsicle.

etix/magick,zuriu/magick,zuriu/magick,rainycape/magick,500px/magick,500px/magick,zuriu/magick,rainycape/magick,rainycape/magick,500px/magick,etix/magick,etix/magick