[WIP] Python 3 support

This changeset adds Python 3 support. I currently target Python 3.2 which ships by default with Ubuntu 12.04 LTS. Unfortunately, [dropped](https://github.com/mitsuhiko/logbook/commit/05ff9f3e12d39fc644cd249e233c20409b17f127) Python 3.2 support, hence it has to go. In any case, at the moment we reduced the required features to that of the logging module anyway.