Server-side encoding/decoding

[sgg]

The client half of the types provided by Brokaw more or less boils down to two things:

1. Command encoding (implemented via ToString Encode)
2. Response decoding (implemented via TryFrom<RawResponse>)

One could extend Brokaw to support server-side connections by implementing (1) command decoding and (2) response encoding.

It might be worthwhile to reconsider the use of ToString as a trait for command encoding as it prevents Brokaw from supporting non UTF-8 messages (tracked in #26)