build.xml improvements [1423]

* HEADS-UP: *all* JARs used by McV are now copied into dist. This is
done so that McV can be run much like the idv ("java -jar mcidasv.jar")
and will only happen after running "ant clean" or building McV for the
first time.

* Class files are now placed into build/antbuild ("antbuild" to avoid
any IDE problems).

* "Incremental" building avoids needless copying/recompiling (when
possible). However, mcidasv.jar and local-idv.jar always have changes
to their manifests and are always rebuilt… :(

* Cleaned up classpath generation.