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.
python 3 doesn't allow attribute names to be bytes.  this variable is used a few lines later as:

    # Add the method to the class under the proper name
    setattr(target_cls, test_method_name_for_dataset, test_method_for_dataset)