Changing extension key type from IRL to IRI.

This is possibly contentious but should probably be addressed.

Although the extension section defines each key as an IRL, [Appendix F](https://github.com/adlnet/xAPI-Spec/blob/master/xAPI.md#conversion-of-statements-created-based-on-version-09) indicates 0.9 extensions should be tagged as IRIs. So as to not break any already-converted extensions, I'd propose this backwards-compatible change to make extension keys IRIs.

Thoughts?