pass flake8 + related tox configuration

flake8 is a style utility which wrap pep8/flake8 and mccabe if availabe.
I have added a tox environnement to easily run it.

The E501 (line too long) error is ignored, it does not play well with
the inlined examples in comment blocks.

To run:

 tox -eflake8
Merge pull request #52 from hashar/pep8-pyflakes

pass flake8 + related tox configuration