Next attempt at input validation
Added data validation checks
Used correct error type
Added missing check
Added Bryan as a contributor
Bug fixed and such
Added @TAXIIMessage.in_response_to.setter idiom
Added support for sets in _do_check
Made a change allowing subscription ID to be None
Allow subscription_id to be None, per the spec
Added support for tuples
Added namedtuple and changed argname
Changed if statement to be more readable
Changed usage to **kwargs
Changed to dict.get(...) in many places
Merge pull request #27 from TAXIIProject/issue_24

Closes #24.