Add support for Tox (http://tox.testrun.org/)

This PR adds support for the [Tox testing tool](http://tox.testrun.org/).

[Tox](http://tox.testrun.org/) is a nice tool for testing Python modules with various versions of Python and various sets of packages.

I thought it would be useful here because it gives developers an easy way to quickly verify that PyStemmer is working as expected on various versions of Python such as 2.6, 2.7, 3.3, pypy, etc.

The `tox.ini` file is the main thing needed, but I also made a change to `runtests.py` so that it exits with a non-zero exit code if any doctests fail. Otherwise, tox will report success even when there are test failures.

If there is a CI system, `tox` is a good command to execute in your CI job to verify all Python versions at once.

And the good news is that the tests are passing on a wide range of Python versions:

```
$ tox
…
  py26: commands succeeded
  py27: commands succeeded
  py33: commands succeeded
  pypy: commands succeeded
  congratulations :)
```

Note that it would be nice for Subversion users to edit the `svn:ignore` property and add `.tox` to it.