Add NewReader to public API.

NewReader allows direct extraction of the embedded JPEG thumbnail found in
CR2 images, without first needing to decompress the thumbnail.

I've started looking into https://camlistore.org/r/1217 again.  I think this is the only change I actually need in cr2.  All the djpeg specific stuff can and should live in a new package, as it will be applicable to CR2 images and large JPEGs.

Note: I don't fully understand what you were going for with the err & err2 originally in Decode().  It disappears as part of this refactor.