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>

tox.ini

@@ -8,7 +8,7 @@ passenv =
 deps =
     -rrequirements.txt
 commands =
-    nosetests -v {posargs}
+    nosetests -v --with-coverage --cover-branches --cover-package=compose --cover-erase --cover-html-dir=coverage-html --cover-html {posargs}
     flake8 compose tests setup.py
 
 [testenv:pre-commit]
@@ -38,6 +38,7 @@ deps =
 [testenv:py34]
 deps = {[testenv:py33]deps}
 
+# TODO pypy3
 
 [flake8]
 # ignore line-length for now