Fixed a bug with movement narration

Players now receive narration of another player's movement out of
the room at the right time, and players in the destination room
now receive "incoming" narration at the right time.
Enhanced comments around some movement code
Re-implemented 'look' action

This commit includes the addition of a look method in the
DungeonUniverse class.
Removed a weird narration phrase
Refined descriptions of rooms

Notably: added a describePlayers method to the Room class that
returns a list of players in the room (from a certain player's
perspective).