added markings

Added marking ability to a STIX package:

...
spec = MarkingSpecificationType()
spec.set_Controlled_Structure("//node()")
tlpmark = TLPMarkingStructureType()
tlpmark.set_color("AMBER")
spec.set_Marking_Structure([tlpmark])
...
stix_header = STIXHeader()
stix_header.handling = Handling([spec])
...