authenticate first
authenticate first

Otherwise, I get the following:

2014/10/28 12:16:54 Got banner:  news.example.com NNRP Service Ready (posting ok) (yEnc enabled).
2014/10/28 12:16:54 Error in setting reader mode: 480 Authentication required for command

Closes #2

dustin/go-nntp,wathiede/go-nntp,diocles/go-nntp