[IMP] mail: improved mass mailing. Correctly take into account email_to, email_cc...

[IMP] mail: improved mass mailing. Correctly take into account email_to, email_cc, partner_to even if no template. Mass mailing is a no-subtype post with additional recipients manually notified. Added option (in technical features) to send to followers, or to directly create a mail_mail. Added some default values in mail_message. Added explicit management of email_from and reply_to. Having a reply_to bypasses the 'model-res_id' in message_id behavior allowing to construct threads. Added explicit recipient_ids field on mail_mail that replaces the recipient_ids parameter in mail_mail.send(). Updated tests accordingly.

bzr revid: tde@openerp.com-20130225164857-i635atksj7riq9nd