Change image-reading backend for speed improvements

I just changed the backend to use `skimage.io.imread` instead of PIL's `Image.open`. That brought the time down from 600s to 3s!

However, I'm not sure what the volume is supposed to look like, so could you compare a few slices of `v = imio.raveler_to_labeled_volume('dir')` using this update to what Raveler is showing to make sure it's right?