To avoid littering the codebase with 'Fix PEP8' commits, we've decided
to reintroduce a treshold so PEP8 fixes can be batched together.

See https://pypi.python.org/pypi/flake8 for more details.  Hopefully,
this will be useful and not annoying.

That way it is picked up for example by vim.

Also output the errors directly now that it's not over 100 anymore.

Upgrading pyflakes meant the number of violations jumped above the
failure threshold.

Also added a few more warnings to the ignore list.

Document the lint file more.

This is to ensure that pull requests fail when they don't obey pep8
enough.