Implemented a jpg_info function that reads JPG files to extract info
about size.

For the time being, the function show_image is not really useful, as
the user needs to know the size of the JPEG image in order to use
it. This can now come from the jpg_info funtion: one can therefore
incorporate images of unkown dimensions.

git-svn-id: svn+ssh://rubyforge.org/var/svn/tioga/trunk/tioga@639 f6f669a6-2b79-499e-8f05-be2b991fc3d0