Add tox.ini for tox (http://tox.testrun.org/)

This adds support for testing with [tox](http://tox.testrun.org/).

Sample run:

```
marca@marca-mac:~/dev/git-repos/terrarium$ tox
...
  py26: commands succeeded
  py27: commands succeeded
```

Note: I know that you have Travis CI set up which I also love. I think it's handy to have both Travis CI and tox set up -- the reason is that tox lets developers test their changes locally across multiple Pythons **without having to check in or push their changes**. Thus, I find Tox super handy for working on Python 3 compatibility, because you can keep iterating on making changes and testing with Tox.