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/unit/progress_stream_test.py

@@ -8,7 +8,6 @@ from tests import unittest
 
 
 class ProgressStreamTestCase(unittest.TestCase):
-
     def test_stream_output(self):
         output = [
             '{"status": "Downloading", "progressDetail": {"current": '