Add datamanager for sending email
Since raa0589b57da0, we have support for two phase commit. One of the main goal of this implementation is to be able to send email transactionally. For that we need an email datamanager. I think it should be a core feature of trytond as we already have configuration option and get_smtp_server in tools.
Here is review17981002 that implements one. The design ensures the transaction integrity by never fails when sending email because any SMTP error will just be logged.