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

[Tox](http://tox.testrun.org/) is a great tool for testing compatibility with various Python versions -- e.g.:

```
marca@marca-mac:~/dev/git-repos/pylibmc$ tox
...
  py26: commands succeeded
  py27: commands succeeded
  pypy: commands succeeded
  congratulations :)
```