Merge pull request #409 from Standard8/fix_reconnect_long_polling

Fix long polling - clear pending requests when a client reconnects

r=@n1k0
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
added comments for further refactoring
fixed display of users list
moved appstatus reconnection event handling to the sidebar app
added comment about fullName filtering
Added app.models.User#isContact attribute support

This helps disambiguating users from contacts in the users list
view, so we know if we should keep or remove a given entry
accordingly.
fixed remaining issues with views
removed addressed XXX comments
simplified UserSet#excludeUser()
fixed usermodel tests