Fix add_file and test_handle_file scripts.

For add_file:

- add_file broke when the arguments were changed to related_ from parent_.

- This commit also adds the ability to add files related by ID. You can not
  specify both a related MD5 and a related ID though.

- Fix a couple of cases where the script was calling sys.exit instead of
  return.

- Finally, fix the actual call to handle_file. It was using parent_ instead
  of the related_ family of arguments. It was also missing passing reference
  by keyword.

For test_handle_file:

- Fix the docstring that contains the definition for handle_file.

- Use AnalysisResult now.