Correct bucket deletion for s3.

* src/dctc/java/com/dataiku/dctc/file/S3File.java: If we don't found
  any file with a bucket name, find if the bucket really exists.
rmdir: If not found or not empty don't try do delete the
directory. Closes: #18.

* src/dctc/java/com/dataiku/dctc/command/Rmdir.java: Here.