Added .travis.yml for continuous integration testing

travis-ci.org is a nice little useful service. 

Once configured, It builds or tests all branches on change, and even pull requests. This service is for free for open source projects.

See http://about.travis-ci.org/docs/user/languages/python/ for more infos. 

To set up, login on travis-ci.org using oauth service of github and activate this project. travis-ci will set up the hook automatically. The file .travis.yml is used to configure, how travis-ci should build and test the project.