Use unittest2 for Python <2.7

Sorry about the noise - made a mess of my git repository!  Reverted commits were intended for different pull request.

This allows the tests to run unchanged on Python 2.6 specifically.

Not sure what Python versions you're targeting but this seems like a quick fix to bring at least 2.6 into the fold (all tests pass on 2.6.5 - I haven't tested others)