Add support for testing with tox

This lets you test across Python versions by running `tox` (after you `pip install` it).

```
marca@marca-mac:~/dev/git-repos/tldextract$ tox
...
  py26: commands succeeded
  py27: commands succeeded
ERROR:   py33: InvocationError: /Users/marca/dev/git-repos/tldextract/.tox/py33/bin/pip install --download-cache=/Users/marca/.pip/download-cache /Users/marca/dev/git-repos/tldextract/.tox/dist/tldextract-1.1.3.zip -U --no-deps (see /Users/marca/dev/git-repos/tldextract/.tox/py33/log/py33-6.log)
  pypy: commands succeeded
```