Handled EmailReference

This pull request handles the EmailReference pointer message which has been described in #4.
So... once an archive which is stored in a BLOB is unpacked, I would expect to get something like:
```xml
<!-- 6d7bd557-4a98-4a64-873d-5b4001169e68.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>
</email>

<!-- 51f4ad70-2c14-4e6e-8cf9-19879af6cc01.xml -->
<?xml version="1.0"?>
<email xmlns="urn:grean:schemas:email:2014">
  <attachment>
    <content>(To be determined.)</content>
    <mime-type>image/png</mime-type>
  </attachment>
</email>

<!-- a0dbba7c-de7a-41f3-9c08-f1980885086f.xml -->
<?xml version="1.0"?>
<email xmlns="urn:grean:schemas:email:2014">
  <attachment>
    <content>(To be determined.)</content>
    <mime-type>audio/mpeg</mime-type>
  </attachment>
</email>
```

If that's the case, in which [form of binary-to-text encoding](http://en.wikipedia.org/wiki/Binary-to-text_encoding#Encoding_standards) should we expect to parse the content?