Pyflakes support

[PyFlakes](https://pypi.python.org/pypi/pyflakes) checks are in.

* `pyflakes` support (just like `flake8` does it), messages are starting with `F`, e.g. `2:1: F401 'os' imported but unused`
* activation levers for _pep8_, _pyflakes_ (implies _pep8_ too), _wip_ and _commit messages_

Preview: https://github.com/greut/invenio/pull/2