Drop logbook in favor of logging

logbook does not play nicely with Ubuntu 12.04's Python 3.2 and most of the
features provided by logbook became part of Python 2.7+'s standard library.
Use Python idioms common to Python 2 and 3
Fix Numpy errors concerning array indexing
Use Python 3 environment in Travis
Add tox configuration

tox is a testing automation tool that sets up virtualenvs for different test
configurations. Install with `pip install tox` and run with `$ tox`. Simple as
that.