Adds a +1 / -1 on displayed messages in Chatter. Those links allow to vote
or unvote for a given messages.
Purpose: being able to build a Chatter, using a ranking based on votes instgead
of date or ids. This will be used in a future vote-based FAQ.
bzr revid: tde@openerp.com-20120918152401-z6m38b1vllz4ckgi