add release index and testing kit for testers

To facilitiate testing and downloading JES development builds,
build-releases.xml creates an index.html file in each release directory
with a list of all available releases (regardless of whether they were
built!) for users to browse. This means you can just rsync the entire
directory to a server somewhere and give people the URL.

It also zips up the tests, demo scripts, and (in the future) other
stuff to create a testing kit ZIP file. This allows users to run the
JES tests on their own platforms.
use lastIndexOf('.') to determine extension

When there are dots in an image's FS path (as exist on most JES release
packages), the SimplePicture getExtension() method treats everything
after the first dot as the extension, not the last. This is because it
was using .indexOf('.'), not .lastIndexOf('.').