Add a script to find out the correct settings for Asterisk behind NAT

(closes issue #13065)
Reported by: tzafrir
Patches:
      sip_nat_settings uploaded by tzafrir (license 46)
      sip_nat_settings_6 uploaded by mvanbaak (license 7)
Tested by: tzafrir, pabelanger, Dovid and moi


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@168265 65c4cc65-6c06-0410-ace0-fbb531ad65f3