Multiple updates and improvements, including:
- added the --state-file option to allow caching of the TAXII poll times (so only new material is retrieved on subsequent polls)
- refactored TAXII client code - now supports fulfillment requests
- allow the MISP server's CA certificate to be specified
- improvements to tests
- support for adding AIS markings to a STIX package (and parsing AIS markings)
- added an option to output Snort/Suricata rules (--snort)
- allow the TAXII server's CA certificate to be provided in a file for verification (--ca-file)