Tool/ContributionOpener: Moved its content (DirectoryFileHandler class) to...

Tool/ContributionOpener: Moved its content (DirectoryFileHandler class) to ContributionTool as this is not a Tool.

As this is only used in ContributionTool, it does not seem necessary to create a separate
module for that.

Also:
  * Fix undefined variable `splitport`.
  * Use cStringIO instead of StringIO (as the rest of ContributionTool code does).