Tox

Implement Tox (http://tox.readthedocs.org/) testing. This tests Python 2.7 and Python 3.3, each with SQLAlchemy 0.8.4 and SQLAlchemy 0.9.0, for four test runs total. If you cherry-pick SQLA 0.9 fixes from another PR, you'll have all tests pass. Run the tests by calling `tox`.