Support exif thumbnails

http://tilloy.net/dev/pyexiv2/tutorial.htm

Two methods of the Image class allow you to manipulate directly the thumbnail data as a string buffer: getThumbnailData() and setThumbnailData(data_buffer).

The deleteThumbnail() method removes the JPEG thumbnail from the file.

The dumpThumbnailToFile(filename) and setThumbnailFromJpegFile(filename) methods allow you to respectively export the thumbnail to a JPEG file and set the JPEG thumbnail from an existing image file.

>>> image.dumpThumbnailToFile('thumbnail')
>>> image.setThumbnailFromJpegFile('thumbnail2.jpg')
>>> image.writeMetadata()
Note that, since the JPEG thumbnail is part of the EXIF metadata, you also need to call the writeMetadata() method for the changes to be written back to the image file after modifying the thumbnail.