Explicit Python 3 support

This adds Python 3.3 to the Travis CI test matrix and Python 3.3 and 3.4 to the PyPI classifiers.

The classifiers are used by services like for example caniusepython3.com and requires.io to check if all dependencies of a project supports Python 3.

I've run the test suite manually on Python 3.3 and 3.4, and added 3.3 to the Travis CI build. 3.4 should be added to the Travis CI build when Travis starts supporting 3.4 in the near future.