s3: Delete concurrency problem when creating a bucket.

Closes #21.

* src/dctc/java/com/dataiku/dctc/file/S3File.java: Synchronized the s3
  object.
Correct the depth.

* src/dctc/java/com/dataiku/dctc/file/FileManipulation.java: Trim the
  begin and the end of the file path.
s3: Clear the bucket if needed.

* src/dctc/java/com/dataiku/dctc/file/S3File.java: Here.
cp, mv, sync: Could set the number of thread with the command line.

* src/dctc/java/com/dataiku/dctc/command/Cp.java,
  src/dctc/java/com/dataiku/dctc/command/ListCommand.java,
  src/dctc/java/com/dataiku/dctc/command/Mv.java,
  src/dctc/java/com/dataiku/dctc/command/Sync.java: Here.
style: enum iterate with a switch.

* src/dctc/java/com/dataiku/dctc/copy/SyncComputer.java: Instead of
  sequential if.
Correct destination path creation.

* src/dctc/java/com/dataiku/dctc/copy/SyncComputer.java: The
  destination directory is created if the destination doesn't exists
  or isn't a file.