tox.ini: Add pep8 environment
test_parser: Make this independent from test environment path

To make this able to run via testr.
test_addrconv: Fix a way to import the tested module

I'm not quite sure how it works with run_tests.sh.
But the current coding doesn't work when running via testr
probably due to different PYTHONPATH.
travis: Run tests via tox

Also, run pep8 in a separate environment.

The way to specify python versions might seem awkward.
(Well, it seems so to me.)
But it follows one of examples referenced in the official
documentation:
    http://docs.travis-ci.com/user/languages/python/
    https://github.com/twisted/klein/blob/master/.travis.yml