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

-----

<sup>*</sup>Mailtrap.io rocks!, by the way. :) 
So... if we get a StorageException with message *The remote server returned an error: (404) Not Found.* we can probably delete the orphaned message from the queue.