reformatted UserSet#comparator, added comment
moved updatePresence() to UserSet coll. where it belongs
Initial rendering of user entries performed once after signin
render users list when current user signs out
UserSet methods for updating presence info
introduced UserSet#excludeUser
refactored & simplified UsersView#render use of child views
UserEntryView to render on presence changed
added test for UserView#render on user presence changed
removed triggering full list of users on user join/left