#4 Added ability to extract thumbnails

[Gene Cash]

I've added the ability to extract thumbnails from a
JPEG file.

I modified parse_tiff() in exif.py to save IFD pointers
on a stack so that it wouldn't miss anything, then
added the thumbnail extraction to the end of it.

It works with all the examples at exif.org, and with my
Olympus C3000Z images.

With this patch, it's trivial to modify exiftool with
another option to extract thumbnails.

[Gene Cash]

new version of parse_tiff()