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/progress_stream.py

@@ -17,6 +17,8 @@ def stream_output(output, stream):
     diff = 0
 
     for chunk in output:
+        if six.PY3 and not isinstance(chunk, str):
+            chunk = chunk.decode('utf-8')
         event = json.loads(chunk)
         all_events.append(event)