Fix UseJava not copying resource files

This is a fix for this (abandoned?) bug: http://public.kitware.com/Bug/view.php?id=13360

In short, the `add_jar()` command in *UseJava* module currently does not copy the resource files to the build directory before compiling the jar, which causes an error on the first time `make` is executed (but not on the second).

This branch fixes the issue, and now the resource files are copied correctly.

This fix is based on the patch, created by **mefoster**, found in the link above.
Actually, I've just changed the lines where the changes were made in the patch, to adapt to the current version.
So, if this is merged, don't forget to give credit to the author of the patch!