As discussed in #86, this adds Python 2.6 support for the tests and CI.

Since 2.6’s stdlib doesn’t support test skipping, we have to resort to using the back port unittest2.

There are a few ways to make it less ugly:

• use unittest2 unconditionally everywhere. has also the benefit of a consistent test environment.
• do the dance once in a helper module (say tests/utils.py) and then just import unittest from there.
• use pytest for skipping