Python 3 support

Following discussion on PR #32, this adds Python 3 support without any external dependencies. Python 2.6 and above and Python 3.3 and above should work; older versions will not.

The tests are passing on Python 2 and 3; I haven't tried the examples.
We don't like guesses ;-)

What happened under Python 2 when a unicode owner was supplied?
What unit tests have been added to explore this?