Create an exitmap module for DNS exit checks

I think exitmap has a lot of infrastructure which could get used for DNS exit checks. And we already run exitmap with different modules for a bunch of our bad relay checks. So, I propose to create a dns check module and we could use the already existing dnssec.py one for running another, related, check to get an overview of all the DNS related problems our exits have.

We should try to keep the output format similar to Arthur's to make it easier to parse both results series.