eventMessageCallback(): room/nick -> node/resource

In non-MUC messages, using "room" and "nick" is misleading.
Fix #105: Format non-groupchat as whisper.

XEP-0045, 7.5:
> Since each occupant has its own occupant JID, an occupant can send a
> "private message" to a selected occupant via the service by sending a
> message to the intended recipient's occupant JID. The message type
> SHOULD be "chat" and MUST NOT be "groupchat", but MAY be left unspecified
> (i.e., a normal message).

Therefore, visual.formatMessage should treat all non-groupchat messages
in a MUC context as private.

(Direct messages will require their own logic, and be visually distinct from
whispers.)