Handled EmailReference

This pull request handles the EmailReference pointer message which has been described in #4.
Why not this?

```XML
<?xml version="1.0"?>
<email xmlns="urn:grean:schemas:email:2014">
  <from>
    <smtp-address>foo@foo.com</smtp-address>
    <display-name>Foo</display-name>
  </from>
  <to>
    <address>
      <smtp-address>bar@bar.com</smtp-address>
      <display-name>Bar</display-name>
    </address>
    <address>
      <smtp-address>qux@qux.com</smtp-address>
      <display-name>Qux</display-name>
    </address>
  </to>
  <subject>Test</subject>
  <body>This is a test message.</body>
  <attachments>
    <attachment>
      <content>foo.png</content>
      <mime-type>image/png</mime-type>
    </attachment>
    <attachment>
      <content>bar.mp3</content>
      <mime-type>audio/mpeg</mime-type>
    </attachment>
  </attachments>
</email>
```

and then the archive would also contain foo.png and bar.mp3...