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>

compose/project.py

@@ -2,9 +2,9 @@ from __future__ import absolute_import
 from __future__ import unicode_literals
 
 import logging
-from functools import reduce
 
 from docker.errors import APIError
+from six.moves import reduce
 
 from .config import ConfigurationError
 from .config import get_service_name_from_net