[IMP] Open Chatter: messages loading on scrolldown

ADD:
-> load chatter messages on scroll.
-> Test case for chatter message on scroll.
-> Changelog.
Task:https://openerp.my.openerp.com/web#id=6729&view_type=form&model=project.task&menu_id=3942&action=327
Pad link : https://pad.odoo.com/p/r.q6QePBp6BFlNfBEg