Add .travis.yml for Travis CI

This PR contains a `.travis.yml` file for use with Travis CI.

Here's a passing Travis CI build for my fork of your repo (passing for py26, py27, py32, py33, and pypy):

https://travis-ci.org/msabramo/pystemmer/builds/5022365

To set up Travis CI to run on your own repo, you just need to do a one-time setup to link Travis CI with GitHub that takes less than 2 minutes, described at http://about.travis-ci.org/docs/user/getting-started/

Then when commits are added to the GitHub repo, Travis CI will automatically run.