Add Python 3 Trove classifier ("Programming Language :: Python :: 3") to setup.py

This will let people and tools (e.g.: https://python3wos.appspot.com/) know that yolk is compatible with Python 3.

Should do a release to PyPI; it seems that the latest code in GitHub works with Python 3, but the last released version on PyPI ([0.4.3](http://pypi.python.org/pypi/yolk/0.4.3); released 2012-02-06) doesn't work with Python 3.