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.
Yeah, antlr is a Java thing IIRC. Given this code was obviously never executing, I wonder if there's a way to rip it out rather than fix it?