Run tests against Python 2.6, 2.7, 3.3, 3.4 and PyPy2

In particular it includes:
- some extension of CONTRIBUTING.md
- one fix for Python 2.6 in tests/integration/cli_test.py
- one fix for Python 3.3 in tests/integration/service_test.py
- removal of unused imports

Make stream_output Python 3-compatible

Signed-off-by: Frank Sachsenheim <funkyfuture@riseup.net>

script/test-versions

@@ -24,5 +24,5 @@ for version in $DOCKER_VERSIONS; do
     -e "DOCKER_DAEMON_ARGS" \
     --entrypoint="script/dind" \
     "$TAG" \
-    script/wrapdocker nosetests --with-coverage --cover-branches --cover-package=compose --cover-erase --cover-html-dir=coverage-html --cover-html "$@"
+    script/wrapdocker tox "$@"
 done