• Add support for Python 3.3+
• Use mixbox library for features shared with python-cybox and python-maec.
• #292 Support organisation_info in STIXCIQIdentity.
• #276 Fix published_datetime on Vulnerability.
• #274 Correctly add TTPs to STIXPackage.
• #269, #271 Make STIXPackage version readonly.
• #266 Handle custom VocabString values even if they aren't registered.