Merge pull request #133 from mreiferson/fatal_errors_133

nsqd: v2 protocol responses/fatal errors
nsqlookupd: introduce V2 protocol (framed)

* eliminate SendResponse() and ReadResponse() in favor of
  always using ReadUnpackedResponse()
fixes after rebase
nsqd: if we error sending heartbeat exit
nsqlookupd: a bit of V2 refactoring to support heartbeats
improved lookup_peer
cleanup
more refactoring