Use Travis CI for continuous integration.

The CI server is available at https://travis-ci.org/stefanbirkner/quaidan

I use Travis because it provides some features that make
working with an open-source project easier.
* It [builds pull request.](http://docs.travis-ci.com/user/pull-requests/)
* The CI server configuration is part of the project. Hence Travis CI can be used by every developer who forks this repository. She just has to activate Travis CI for her clone.
* It builds and tests Quaidan with different versions of Python (2.6, 2.7, 3.2, 3.3 and 3.4).
* I can install an Apache HTTP Server with mod_proxy_balancer as endpoint for the tests.