Add configuration for Travis CI.

The configuration file has been verified by `travis-lint`.

I don't mean to replace Jenkins with Travis (at least not immediately), but Travis has "pull request testing" [in the works](http://about.travis-ci.org/blog/announcing-pull-request-support/), which is very interesting, and would work much more smoothly than setting up Jenkins to test all feature branches.

See also:
- http://about.travis-ci.org/docs/user/getting-started/
- http://about.travis-ci.org/docs/user/languages/python/