MERGE] odoo bus, im_chat and im_livechat

Add a generic bus for instant communication based on postgres LISTEN/NOTIFY and HTTP comet.
Both threaded and gevent greenlet mode are supported. Chat should now work on every platform.

im_chat improvements
- proper support for multiple windows
- present, away and offline status
- improved data model for multi user chat session

im_livechat improvements
- standard css js assets are now used
- qweb templates are now used instead of jinnja
[FIX] website snippet editor initialisation