Implement io.WriterTo for Message

[lthibault]

Partially addresses #259.

@zenhack ReadFrom is a bit tricker to implement because Decoder.Decode returns a *Message, and there's no obvious place to hook into. I'll leave that for a separate PR, but any insight would be most welcome.

[zenhack]

Re: ReadFrom, you should just be able to assign the result to the receiver's referent, no?

func (m *Message) ReadFrom(...) ... {
    ...
    msg, err := dec.Decode()
    ...
    *m = *msg
    ...
}