add a basic plugin system

The JES plugin system, as it stands, is basically just a fancy way to
install extra JAR's full of Java and Python libraries. It provides
menu options for "Install Plugin" and "Manage Plugins," which let you
copy in plugins.

There's also a Plugin Creation Kit included in the source code
repository, which you can copy to make a new plugin. Just edit a couple
of files, add your stuff, run 'ant', and it creates a plugin for you.
add system and built-in plugin directories

The system plugin directory allows administrators to add plugins that
students can't directly (or accidentally) remove, and the builtin
directory will allow plugin developers to ship copies of JES that
include their plugins.

This commit also includes documentation for how the launchers treat
plugins.
allow JES's name to be customized

In many places, you can now override the name "JES" from the Ant file.
(It does this by reading the name from JESVersion.) This is not
everywhere, many parts of JES still have "JES" hardcoded. This is just
enough to allow users to create custom builds of JES, and give them
identifiable names.

(The names do have to have "JES" in them, though.)
add plugin system to changelog