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.
I was trying to avoid a dependency on six, as genty currently has no dependencies (although this patchset introduces a dependency for py2.6).

Six could make some of these things a bit cleaner - do you feel strongly that we should add this dependency?