HTML5: create event bus, not finished yet

A global event bus to improve the structure of events in the
HTML5 client.
HTML5: last view, chat, migrated to use the event bus
HTML5: remove events emitted on Connection, now emitted on the event bus

And removed duplicated events for 'connected' and 'disconnected'.
Merge pull request #161 from daronco/html5-event-bus

HTML5 client - events
user leave now uses the html5 bridge
manage UserModel in UserCollection