docs: Fix syntax error
tox: Add tox config file

This allows for testing across multiple environments, including Python 3 in
the not too distant future. The instant bonus is that this allows for testing
the docs build at Travis so I don't make more releases like 0.18.2 with broken
docs.
travis: Use tox to run both tests, docs build, and flake8 linting
docs: Add link to coveralls.io
docs: Update testing docs