Rethink email integration into this plugin

@mukil Instead of integrating the send mail option into this plugin, I would actually much more prefer to exclude the send mail code from the sign-up plugin, but create a new plugin just for sending out emails (do one thing well!).

This extra sendmail plungin could be re-used in various contexts by other plugins. It would extend the user type with an email-address, as well as it could provide a TopicType E-Mail (reply-to, recipient, subject, cc, bcc, body - and possibly attachments, which I would model as an extra assoc type.). We could then also use it for users to send-out emails.

For mails initiated by the system or other plugins, e.g. by the sign-plugin, each plugin could have it's own configuration options for sender, recipient etc.

What do you think?