Run tests with different SQLAlchemy versions

The test suite should be run for all possible combinations
of Python and SQLAlchemy versions we claim to support, which
are Python 2.6 and 2.7 and SQLAlchemy 0.7.x (old stable) and
0.8.x (mainline stable).

py26/py27 tox envs install the latest stable version of SQLAlchemy
(currently, 0.8.x branch), so two additional tox environments added
to run the test suite with SQLAlchemy 0.7.x (for both Python 2.6 and 2.7).

Change-Id: I50c7009d8b808ce3bbb1f0a27c50f5cb5116cdb3