Refactored the error handling and processing of multiple XML
documents.
* Added support for multiple exit status codes.
* Validation exceptions do not abort further validation attempts
  on files during batch validations. Validation only halts for
  the file which raised the exception.
* Added ValidationErrorResults class which are used to convey
  exceptions that occurred during documentt validation. This
  is only used by stix_validator.py at the moment. Not sure if
  validate() methods will use this or continue to raise
  exceptions.
* Updated docstrings where I noticed outdated comments.