[FIX] mail: various fixes

[FIX] mail: various fixes
Mass mailing: emails due to mass mailing are not queued instead of sent immediately.
As most of cases go through mail_followers._notify(), a new argument is added that
allows to control the force_send (like what is done in email_template).
Chatter: better display of recipients when having a message_post due to mass mailing
(not always displaying logged a note).
Notified people: when having no subtype, just avoid notifying the followers but
take into account specified recipients (partner_ids).

bzr revid: tde@openerp.com-20130606102437-43s4xp96b5wa3r1q