Continuous integration support

Thought you guys might appreciate this - continuous integration for PySB. Every time you push the repository to Github, the unit test suite is executed and an email is sent if it fails. This uses a service called Travis-CI (see https://travis-ci.org/alubbock/pysb for example).

All you need to do is sign in to Travis CI with your Github ID and add the service hook, which takes 2 mins. Instructions here: http://about.travis-ci.org/docs/user/getting-started/

Don't forget to change the image URL in README.rst to point to your own repo's build status once you get set up.

Enjoy!