The name of the package will be normalized by `setuptools` anyway and it will
most likely appear as dash-separated on PYPI.[1]
This change also helps to keep the distinction between the PYPI package name
and regular python packages (the directories with __init__.py files).
[1]: https://mail.python.org/pipermail/distutils-sig/2011-August/017936.html
All versions of Python that flake8 supports (2.7, 3.4+) have full
support for the with statement. There is no need to add the import.
Slightly simplifies the example be removing unnecessary boilerplate.
Previously the `X` wasn't further explained in the documentation on how to
register a plugin. This plugs the hole at least for checking plugins.
See also: #183
