Dns helper v2

This is the second version of #102

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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.