Run tests with Travis CI

With [Travis CI](https://travis-ci.org/) enabled for your repository, this change will:

1. Cause tests to be run in Python versions 2.5, 2.6, 2.7 and 3.3 with every push.
2. Add a dynamic badge to the README with the latest build status of the master branch.
3. Add a green or red bar to any pull requests you might receive depending on the outcome of tests (see this patch).

Right now, Travis is only enabled for [my fork](https://travis-ci.org/dokterbob/surlex). Hence I recommend you [enable Travis](http://about.travis-ci.org/docs/user/getting-started/) (step 1 and 2, step 3 is this patch) before merging this patch so the result should be immediately visible after the merge.