add a Mac OS X .app build

This was kind of complicated to pull off. Instead of Oracle's provided
AppBundler, I'm using an open source tool named JarBundler with a
custom, JES-specific shell-script application stub instead of the
standard Objective-C stub that reads information from the Info.plist
file.

Now, "ant release" generates a JES.app bundle, and a zipped-up version
(jes-@version@-macosx.zip) that extracts directly into the app when you
unzip it (and even has a correctly set execute bit!).