Add the userid in emails their message id

Adding the userid in the [`messageId`](https://github.com/oaeproject/Hilary/blob/master/node_modules/oae-email/lib/api.js#L375) would allow us to identify the user object in OAE that received an e-mail by its ID. 

This is a quick fix until we have globally unique email addresses and can lookup users by emails.
Should also include documentation about the change in the documentation block