Hackebrot integrate pytest in setup

I implemented a `PyTest` command class in `setup.py` similar to http://pytest.org/latest/goodpractises.html#integration-with-setuptools-test-commands

Currently `python setup.py test` still uses `unittest` which means some of the re-implementations are not run at all. By changing this the documentation changes to `CONTRIBUTING.rst` of #310 are correct again.

Additionally I removed `pytest.ini` to address #333. It stopped us from running only specific tests as for instance `py.test tests/test_generate_file.py`. More importantly the suite is not executed twice any longer and the coverage is not calculated when running tests only.

Your feedback is highly appreciated! :kissing_closed_eyes: