[IMP] bus: re-factoring of bus.bus (Longpolling and CrossTab)

The purpose of this change is to make the code clearer and testable.

In this change, the 'tab_manager' static object was merged with the bus
cross tab.

Cleaning was done to clearly define private and public functions as well
as handlers. The methods are documented and the constants are now defined
on the class. The bus use the service behavior with 'trigger_up'.

'bus.CrossTab' who extend 'bus.Longpolling' are instantiated by the
bus service.

The class is always instantiated with a parent, or root in the case of the website
(im_livechat), to use the ajax and localstorage services. So the behavior, perhaps
logger or redefined by the parents.