[issue #7] Introduce support to Python 3.x

- Add Six package, a simple utilities for wrapping over differences
  between Python 2 and Python 3
- Encapsulate compatibility methods in 'support.py' module
- Introduce unit tests for 'support.py' module

Review needed for two failing tests on Python 2.7.x and 3.x:
- test_unknown (pinject.locations_test.GetClassNameAndLocTest)
- test_unknown (pinject.locations_test.GetTypeLocTest)