Apip Accessibility data model and de/serialization

The addition of all of the data structures within the apipAccessibility element introduced in QTI 2.1 Final. A related capability that is also included is the ability to control the output namespace-profiles used for serialized QTI/APIP documents via the new QtiProfile enum. 

This is a semi-duplicate of previous pull request #32, though now on a non-changing branch to keep the pull a stationary target.  If github had allowed me to simply alter the previous request, I would have done so.