Handled EmailReference

This pull request handles the EmailReference pointer message which has been described in #4.
FTR, I just made another test using Node.js to write XML files with large attachments.

Sample.js:

```javascript
// npm install xml-writer
require('fs').readFile('Xyz.mp3', function(err, data) {

    var xml = require('xml-writer')();
    xml.startDocument();
    xml.startElement('email'); 
    xml.writeAttribute('xmlns', 'urn:grean:schemas:email:2014');

    // (The indentation is intentional.)

    xml.startElement('from');
      xml.startElement('smtp-address');
        xml.text('foo@foo.com');
      xml.endElement('smtp-address');
      xml.startElement('display-name');
        xml.text('Foo');
      xml.endElement('display-name');
    xml.endElement('from');

    xml.startElement('to');
      xml.startElement('address');
        xml.startElement('smtp-address');
          xml.text('bar@bar.com');
        xml.endElement('smtp-address');
        xml.startElement('display-name');
          xml.text('Bar');
        xml.endElement('display-name');
      xml.endElement('address');
    xml.endElement('to');

    xml.startElement('subject');
      xml.text('Test');
    xml.endElement('subject');

    xml.startElement('body');
      xml.text('This is a test.');
    xml.endElement('body');

    xml.startElement('attachments');
      xml.startElement('attachment');
        xml.startElement('name');
          xml.text('Xyz');
        xml.endElement('name');
        xml.startElement('content');
          xml.text(data.toString('base64'));
        xml.endElement('content');
        xml.startElement('mime-type');
          xml.text('audio/mpeg');
        xml.endElement('mime-type');
      xml.endElement('attachment');
    xml.endElement('attachments');

    xml.endDocument();
    console.log(xml.toString());
});
```

Usage:

```
node sample.js > sample.xml
```

Then, I uploaded the XML to the BLOB and used the technique described in the previous [comment](https://github.com/GreanTech/Qaiain/pull/18/files#r14179272) to capture the email.

It seems to be working decently here :)