Added docstring for Simulator

- This shows up when doing py.test --fixtures
Using pytest.mark.parametrize in test_ensemble
Now using tox for testing

This makes it easy to know if your changes will work on Python 2.6,
2.7 and 3.3 without having to push up to travis-ci.

Now, if you pip install tox, you don't need anything else installed;
you can just run `tox` and it will set up a 2.6, 2.7, and 3.3
environment for testing. tox will work just fine in a virtualenv too,
so no changes required if you already have a virtualenv set up.

- .travis.yml uses tox now as well