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.
The ```dict``` creation seems extraneous. Could you do this -

```
entries = target_cls.__dict__.items()
for key, value in entries:
  ...
```