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>

tests/integration/cli_test.py

@@ -275,7 +275,7 @@ class CLITestCase(DockerClientTestCase):
         self.command.base_dir = 'tests/fixtures/user-composefile'
         name = 'service'
         user = 'sshd'
-        args = ['run', '--user={}'.format(user), name]
+        args = ['run', '--user={user}'.format(user=user), name]
         self.command.dispatch(args, None)
         service = self.project.get_service(name)
         container = service.containers(stopped=True, one_off=True)[0]