Incorporate simgrid-java in simgrid-java/.

In order to keep the history of the simgrid-java repository,
the following procedure was used:
$ git remote add -f java /path/to/simgrid-java.git/
$ git merge -s ours --no-commit java/master
$ git read-tree --prefix src/bindings/java/ -u java/master
$ git commit -m 'Incorporate simgrid-java in simgrid-java/.'