Dns helper

As a  followup to our discussion on pox-dev (http://lists.noxrepo.org/pipermail/pox-dev-noxrepo.org/2013-November/001188.html) I'd like share the current state of my modifications/additions.  I wrote some of my doubts in comments.  Moreover, I haven't tested the new dns_spy module at all.

Please, review.

Thank you.