Take care for db size when smtpclient module installd

Hello,

After reading bug https://bugs.launchpad.net/openobject-addons/+bug/396125, it seems that its better to keep the history for sent emails. in openerp server. but in other ends it may increased the size of the database quickly if this module is used the send thousands of email / day.

Better solution is to allows users to save the email on disk, with related information and also delete the history after some period of times i.e 10, 20, 30 days automatically.