Added validation to TaxiiXml beyond what the schema checks

TaxiiXml can now validate a MessageType. In addition to the schema
checks, it also validates an Invalid Response Part Status
Message.  Other non-schema checks can be added as well.

Because TaxiiXml does validation checks above and beyond the schema
checks, validateFast() and validateAll() are now the preferred
way of doing validation; validation associated with marshalling
and unmarshalling was removed, and documentation was added
showing how to use validateFast() or validateAll() instead.

Added StatusMessages and StatusDetails constants.
Added more unit tests.
Added StatusMessages.createInvalidResponsePart() utility method
added another unit test for a bad invalid response part