Python 3.3 support

Hi,
I ported this over to Python 3.  I have tested for backwards compatibility with Python 2 and all seems well although I havn't tested it extensively so it may need some more investigation.

I chose to use importlib for 'relative' style imports.  Forgive my naievity if this was a bad choice, its the first time I've run into the differences between the Python 2 and 3 module import handling.