added input validation on service_type property of DiscoveryResponse.ServiceInstance
added tuples for all message types, status types, and actions
fixed comment
Merge pull request #23 from bworrell/master

Added tuples for global constant categories and input validation on service_types property of DiscoveryResponse.ServiceInstance class

Just as a note, the primary purpose of the tuples is to support input validation.
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.