Handling of EmailReference messages

This pull request handles the EmailReference pointer message (related to #4) as discussed in #18.

It has been tested with:
* [Mailtrap.io](https://mailtrap.io/)<sup>*</sup> for attachments of small size ~1MB
* the technique I previously described [here](https://github.com/GreanTech/Qaiain/pull/18/files#r14179272) for attachments of size ~20MB

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<sup>*</sup>Mailtrap.io rocks!, by the way. :) 
Moving the `handle` function to the `Mail` module sounds like a good idea, but *you're* introducing the function in this Pull Request (PR), so you're allowed to have a stronger opinion of its design than me :)

Still, if it's difficult to move, I'll also be happy dealing with this in a separate PR.