Support Python 2.6, 2.7, 3.3, 3.4, and pypy.

Add testenvs to tox.ini for these configs.

Adds a dev dependency on unittest2 for python 2.6.
Adds a dependency on ordereddict for python 2.6.
Could you use [Six] (https://pypi.python.org/pypi/six) or [types] (https://docs.python.org/2/library/types.html)?