Change to new go-nsq consumer and config styles

The current head of go-nsq (post 0.3.7 release) has changed the reader
files/methods to consumer, and moved where config is done. So just
moving the usages around to match:

- NewReader is now NewConsumer
- NewConsumer must be passed a config object from NewConfig, and config
  changes done before NewConsumer instead of being done on the returned
  object
- ExitChan changed to StopChan