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.
Do these imports need to be here?
If so, why do we have an import loop?
(not related to the Py3 change, but I am curious...)