Add "Post Modules", a serie of modules whose purpose is to track exchanges in/out of ERP5.

Currently, users write Mail Messages (usually as a HTML content), and then send the Mail Message. The RFC5322 version of the email (what is given to MailHost and sent over the network) is generated on the fly and never stored anywhere. About Letter, no output is generated. It means that exchanges are not tracked, and if an Event is sent 2 times, we don't know their exact content on both times.

Posts have the purpose to fix this issue, by creating objects storing the content of the sent Events **exactly as they are transmitted out of ERP5**. In exemple, sent Mail Messages will be converted to a  multi-part message, stored as a Internet Message Post, which content will be transmitted as such to MailHost. For Letters, their content can be processed (by erp5_corporate_identity for exemple), then converted to PDF, and stored as Letter Post. We can then imagine actions/alarms to gather all exportable Letters Posts and mail them to a printer.

Posts follow their own workflow, shared in between all post types, which allow to track which posts are in the sending queue, which have been sent and which have been received. It must be easy to know when an event has been sent several time, and retrieve the content of each of these sendings. Design of the post modules/workflow should also allow an easy customization (for exemple, we may want to send all emails by bulk, instead of  1 by 1, in order to set up mass-emailing policies).

/reviewed-on !532