add a build.properties file for the Ant script

If you want to store per-machine settings like your key information,
you can now do that by writing them all in build.properties. This file
is .gitignore'd, and loaded at the beginning of the build script before
defining any other properties (except the ones defined with -D).
expand release process documentation

Now, the release process is fully enumerated in
working-on-jes/building-releases.md, including how to configure your
workstation for JAR signing and Windows installer building.
exclude Jython's $py.class files

I forgot that Jython compiled its own standard library too.