Converted NntpResponseDecoder into a proper FrameDecoder to handle multiline.

* NntpResponseDecoder now requires a CommandPipelinePeeker to know what type of
  response it's decoding.
* NntpResponse.ResponseType enum now constructs instances of responses. Yay.
* Hopefully these changes will make it easier to implement multiline parsing,
  and more importantly, unit testing.
More prep work on multiline decode support. NntpResponseDecoder is now testable.
Multiline decoding support + tests.