Refactoring check messages

* all the API calls are made outside the check functions (easier to test)
* checking enhancement:
 * bullet points must start with `*`
 * signatures cannot appear before bullets
* exception messages are sent back to the hook
* since comments are made in the commit itself, the commit sha is not repeated there
* high code coverage

#### ⚠ backward incompatible changes ⚠

* `invenio_kwalite.main` is no more, use `gunicorn invenio_kwalitee:app` instead (See 3f11901)
* the headers config is no more, use `ACCESS_TOKEN` instead (See e177a19).
* dropping support of Python 2.6 (only the tests are using non-2.6 features)