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, would be happier to lose changes that don't need to be there for Py3. Either than, or let's make them sane.
Agreed on antlr/PyANTLR/etc *sigh*