Make runtests.py exit with a non-zero exit code when doctests fail

git-svn-id: svn+userv://snowball.tartarus.org/snowball/trunk/pystemmer@562 633ccae0-01f4-0310-8c99-d3591da6f01f
Add a tox.ini for tox (http://tox.testrun.org/)

git-svn-id: svn+userv://snowball.tartarus.org/snowball/trunk/pystemmer@563 633ccae0-01f4-0310-8c99-d3591da6f01f
Add .tox to .gitignore

git-svn-id: svn+userv://snowball.tartarus.org/snowball/trunk/pystemmer@564 633ccae0-01f4-0310-8c99-d3591da6f01f