nsq_tail: mv stdout logging to stderr

[jehiah]

This fixes a regression from #957 which added a fmt.Printf line which outputs to stdout. stdout is used exclusively by nsq_tail to output data, and logging should be on stderr via the log package.

$ nsq_tail --topic=internal_request_metrics --channel=blah#ephemeral --nsqd-tcp-address=127.0.0.1:4150 2>/dev/null
Adding consumer for topic: internal_request_metrics

[kpurdon]

👍

[mreiferson]

whooops

[ploxiln]

thanks 👍