Add Travis-CI support.

[Travis-CI](http://about.travis-ci.org/docs/) is a continuous integration server for open-source projects. This pull request adds support for Travis-CI. Creating a Travis-CI account (with your Github credentials) is necessary. Then, we can add a build image to the README with this rst markup:

    .. image:: https://travis-ci.org/davidedelvento/qc.png
       :target: https://travis-ci.org/davidedelvento/qc