Fix channel_at; implement loaded item, etc.
Use JSON errors for auth failure.

Also add a DELETE for playlists, but this won't go active until a few
commits down.
Add RESET code.
Add ClearChannel command; rationalise commands hierarchy.

Commands operating on channels now have a common subclass and all commands
now inherit from Command properly.
Implement the rest of ClearPlaylist.

Next up is to interpret the log message response.
Add LOG_MESSAGE.

Also adds a new leg to the loaded_item test, for empty players.
Allow :empty as a load state.

:empty is now also the default load state.