Add tox.ini

This adds support for the [tox](http://tox.testrun.org/) testing tool, which lets you run your tests across a bunch of virtualenvs with different versions of Python.

Note that there are some failures, the biggest being several instances of `"RuntimeError: Attempted to populate globals twice"`.

```
$ tox
...
ERROR:   py26: commands failed
ERROR:   py27: commands failed
ERROR:   pypy: commands failed
```