Add tests/conftest.py for py.test

This does for py.test was tests/nose_plugin does for nose, but it's
nicer because there is no plugin that needs to be registered in setup.py
(or no custom test runner to force the use of the plugin)
Make some test classes subclass unittest.TestCase

so that py.test invokes their setUp methods
tox.ini: Use py.test and don't use setup.py develop
.travis.yml: Use py.test