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. :) 
Let's start with that - it's a bit too many arguments to my taste, but it's a decent starting point, so let's start with that, and see what we can learn from this direction...

Just thinking out loud... Such a `getMessage`/`deleteMessage` pair almost sounds like some sort of context in itself. In C#, many people would model it with a `using` block. In F#, perhaps it could be modelled as a monadic type...

However, let's not go there right now... let's get it working first - it's not *that* complicated, after all.