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.
As I said, given the shadowing, it was impossible to have a call that passed txt to the implementation succeed, so let's lose the txt argument :-)