Added XML schema files
Modified schema to have version info
Added test for 1.1
added 1.1 functionality to a number of classes
Changed idiom. Closes #25.
Significantly expanded tests. Added tests to ensure that
new features in TAXII 1.1 are supported, and that
when new TAXII 1.1 features are used and the message
object is serialized to 1.0, appropriate warnings/errors are
printed/raised.
Added more support for TAXII 1.1