Paver: fail if pylint/pep8 violations threshold is breached.

It's more intuitive to check if something exists, than to check if it doesn't exist. Therefore, it's better to name your variables to follow that idea. Instead of having the option set a variable called `ignore_violations_limit` to True, it would be more intuitive to have this option set a variable called `limit_violations` to False. That way, the code that checks for this can simply be `if limit_violations:`.