Change Travis config to use tox and report coverage

[sergei-maertens]

• Make 'python setup.py test' working as expected
• Enabled coverage tracking in tests
• Let Travis use Tox instead of manually specifying the build matrix

Coverage results are pushed to Codecov and Coveralls. You would probably need to enable this repository there: https://codecov.io/, https://coveralls.io/. Codecov in general is a bit stricter than Coveralls (measuring branch coverage as well).