Dns lua

This patchset adds 'dns.rrname' field to the list of exported to lua. This allows to do some scripting on lua query. One can think to DGA and data evasion.

PR builds:
- PR build: https://buildbot.openinfosecfoundation.org/builders/regit/builds/16
- PR pcaps: https://buildbot.openinfosecfoundation.org/builders/regit-pcap/builds/14

BytesToString allocates memory. I don't think this is freed anywhere.