game/commands/standard/standard_set: Implement UnknownResponse

Finish up implementation of the UnknownResponse. This message is sent when
a client sends something the server doesn't understand.
- This includes a modification to the parse command, which now accepts an
  initiator of the parsing.
- Update the respsective standard_set unit tests.
- Update the ICommandSet.parse method with an optional initiator
Merge pull request #6 from matt-jordan/unknown_response

game/commands/standard/standard_set: Implement UnknownResponse