Extensions Clarification

Handles Issue #553 

After discussing some conformance requirements with @andyjohnson , it seemed like there needed to be a bit more clarification on the term map as well as a requirement about not having empty extensions.
For clarity, I'd like to prefix this with "When present, the Extensions object SHOULD*". 

I'm not actually sure about this new requirement in general though. I'll comment in the issue on that point. https://github.com/adlnet/xAPI-Spec/issues/553