Handled EmailReference

This pull request handles the EmailReference pointer message which has been described in #4.
Part of the work of interpreting the BLOB is to read the manifest file, and produce a record that ultimately can be used to populate a [MailMessage](http://msdn.microsoft.com/en-us/library/system.net.mail.mailmessage.aspx) instance.

In order to add attachments to a MailMessage instance, it's necessary to be able to create an [Attachment](http://msdn.microsoft.com/en-us/library/system.net.mail.attachment.aspx) instance for each attachment.

While you *can* create an Attachment instance with a file name, that would require Qaiain to unpack each attachment file from the archive and place it in a temporary folder, and then make sure that it's properly cleaned up when the email has been sent.

A better option, it seems to me, is to use one of the Attachment constructor overloads that accepts a Stream.

This means that the `Attachment` record type should contain the 'value' of the attachment in a format that enables us to produce a Stream from it. Perhaps the type of the Content should be a byte array; perhaps it should be a Stream; perhaps it should be something else entirely...

BTW, our `Attachment` record should probably also have a `Name` field...