Provisional Python 3.3 support?

This commit makes the test suite pass consistently on Python 3.3 (for me at least). I noticed that, unlike for 2.7 and 3.2 where the parameter string generated by ScopeURI was consistent, Python 3.3 had the order change arbitrarily between test runs. My solution was to sort the parameters.

I hope you find my re-implementation of ScopeURI.__str__ reasonable, though please feel free to reformat / rename variables / re-implement / etc as appropriate.

I ran tests on Python 2.7, 3.2, and 3.3 on Ubuntu 12.10. I don't have other python installations that I can test against.