Enabling TAXII Service to Communicate over TAXII 1.1

Imported a newer version of libtaxii. Modified polling and inbox functions
to attempt messaging in 1.1 if 1.0 is unsuccessful. Added a config option
to connect without using HTTPS. Modified execute_taxii_agent to pass an
inferred source to import_standards_doc to prevent events from being created
without source names. Converted content_bindings to strings prior to
comparison in parse_content_block.