Add hacky script to symlink JAR files for Fiji

This is nice for development, and IMHO easier to use than

  mvn -Dimagej.app.directory=$FIJI_DIR -Ddelete.other.versions=true

which is overzealous about copying and deleting dependencies.

Once the symlink is in place, you can rebuild as much as you want
and the Fiji installation will always point to the newest build.