cosmetics: PEP-8 conventions.
requirements: Loosen dependency on argparse to avoid CI failures.
travis: Remove deprecated --use-mirrors from pip
travis: Allow external python packages to satisfy pip requirements.
{travis,requirements}: Allow CI failure with Python 2.6.
travis: Kill --allow-external from pip command line.
travis: Fix yaml syntax.
requirements: Bump requests version to work with Python 3.4. Closes: #241.
travis: Test the project also with Python 3.4.
travis: Disable tests with Python 3.4 for the moment.

We need to wait until there is a fix for

    https://github.com/travis-ci/travis-ci/issues/1989
travis: Try to re-add Python 2.6 testing.

We still allow tests with Python 2.6 to fail, though.