Challenge util

I will merge #129 first and when I do, I will merge the highest tox coverage possible. (The goal is that it will monotonically increase and eventually force 100 percent code coverage.)
https://travis-ci.org/letsencrypt/lets-encrypt-preview/jobs/43991540#L1218 answers my question. Interesting idea, @jdkasten. We just have to remember to rise the bar consecutively.