Testing on travis-ci

If you merge this pull request and activate travis-ci, each commit and pull request will be automatically tested on Python 2 and 3 on Ubuntu.

Currently the tests fail because of two issues:
* On Python 3 there is the `long` `NameError` which can be seen in the log on travis-ci [here](https://travis-ci.org/cdeil/ad/jobs/11048913#L108)
* The `doc/Makefile` doesn't work because it runs latex (see [here](https://travis-ci.org/cdeil/ad/jobs/11048911#L116)). I could install latex of course on travis-ci by adding an `apt-get install` to `.travis.yml`, but is this really absolutely necessary to generate the html docs?