Integration testing of nominal cases

Here are some basic tests for the current use case of kwalitee. Before refactoring too much, I wanted to not break it too badly. This PR is mostly a call for a quick review.

* `ping` event support
* `pull_request` event support as previously (hopefully)
* flake8 the code
* giving the exception back to the caller (instead of the console)
* travis friendly: https://travis-ci.org/greut/invenio-kwalitee