clean up some Ant task descriptions

One of them was wrong, and some others were missing periods or phrased
in declarative form instead of imperative.
move classes into a .jar and start signing it

Due to new IT policies at the US Military Academy at West Point, all
JAR files shipped with JES must be signed. So, all the Java classes
have been moved into a JAR, and all the JAR's are now signed by the
build script. Whenever you make a release and set -Dsign=true (and
set -Dsigning.key=key alias), it will make signed copies of the JAR
files, and include those in the release.

(Key generation is not controlled by the build script, so you can have
all sorts of fun with keys and certificates. I'll document how you
generate keys in a minute.)