Mail statistics are now stored onto a separated object (mail.mail.statistics), allowing to
handle emails separately from statistics (among other removing mail.mail entries while keeping
statistics).
Everything linnked to opened/replied/bounce is not managed by mass_mailing, removed added code
in mail module.
bzr revid: tde@openerp.com-20130913115408-322cyjipdg680as6
Thibault Delavallée