Add nsupdate(1) support

The 'nsupdate' protocol is used to submit Dynamic DNS Update requests as
defined in RFC2136 to a name server using the 'nsupdate' command line
utility part of ISC BIND.  Dynamic DNS updates allow resource records to
be added or removed from a zone configured for dynamic updates through
DNS requests protected using TSIG.  BIND ships with 'ddns-confgen', a
utility to generate sample configurations and instructions for both the
server and the client.  See nsupdate(1) and ddns-confgen(8) for details.
Merge pull request #3 from droe/feature/nsupdate

Add nsupdate(1) support