Enable continuous integration with travis-ci.

As the number of contributors increase, it becomes more important to make sure that everyone is able to get started quickly on a clean slate, where 'clean' is defined by the unit tests and integration tests. It also avoids "but it runs on my machine!" scenarios. This pull request adds a travis configuration file which instructs travis-ci to run the tests. A badge indicating the status is included in the README. [Rails](https://github.com/rails/rails) is a great example of a project using travis successfully.

- [travis-ci](https://travis-ci.org/)
- [continuous integration](http://en.wikipedia.org/wiki/Continuous_integration)
- [current build status](https://travis-ci.org/jimjh/kafka-python/builds/12037200), from my fork

Note that you will need to enable github to notify travis whenever a push is made.

At this point, it appears that py2.6 doesn't work too well with `expectFailures`, and that some of the integration tests are failing. I would like to have your help with these.