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. :) 
You are probably right that the correct place to handle this is in the `handle` function itself, but I'd like to get that function under test, then.

As given at the moment, the `handle` function closes over the `blob` value and the `send` function. If, instead of closing over them, it took them as parameters, it would be almost testable. If we also change the input so that instead of passing a `CloudBlobContainer`, it would pass another function that returns a `string`, it would be testable.

Something like this:

```F#
let rec handle (getMessage : string -> string) (send : EmailData -> unit) msg =
```

(Not that I'm suggesting that we should have explicit type annotations on the function - the above is just to illustrate what I mean...)