Use unittest2 in Python versions < 2.7
Add unittest2
Find test modules with TestLoader.discover()
Use --failfast regardless of the Python version
Remove unused module-level variables
Remove unused imports
Update module docstring
Import test.unittest from the top level of the project

This fixes the "Attempted relative import in non-package" error.
Use assertRaises() as a context manager
Merge branch 'use-unittest2-before-2.7'