Use embedded thumbnail from large JPEG images

```
Well, this is not an issue. It's a request for enhancement.
Many JPG files (particully the big one), already have an embedded thumbnail in 
the EXIF.

Would you please add a function to get it? so that the thumbnail generation 
will be very very fast.


```

Original issue reported on code.google.com by `pcman.zh...@gmail.com` on 25 Dec 2014 at 9:47
