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>

Dockerfile

@@ -66,8 +66,8 @@ WORKDIR /code/
 ADD requirements.txt /code/
 RUN pip install -r requirements.txt
 
-ADD requirements-dev-py2.txt /code/
-RUN pip install -r requirements-dev-py2.txt
+ADD requirements-dev.txt /code/
+RUN pip install -r requirements-dev.txt
 
 RUN pip install tox==2.1.1