Handled EmailReference

This pull request handles the EmailReference pointer message which has been described in #4.
FTR, I could successfully Base64-encode a 1920x1080 image in Node.js:

```javascript
var fs = require('fs');
fs.readFile("input.jpg", function(err, inputImage){
    var base64Image = inputImage.toString('base64');
    console.log(new Buffer(base64Image, 'base64').toString('base64'));
});
// -> /9j/4AAQSkZJRgABAQAAAQABAAD//gA7Q1JFQVRPUjogZ2QtanBlZyB2MS4wICh1c2l ...
```

Using the Base64-encoded string, it is possible to create a Stream from its equivalent 8-bit unsigned integer array:
```f#
let bytes = Convert.FromBase64String("/9j/4AAQSkZJRgABAQAAAQABAAD//gA7Q ...")
use stream = new System.IO.MemoryStream(bytes)
// Do something with the stream object.
```

This should work with the [Attachment](http://msdn.microsoft.com/en-us/library/system.net.mail.attachment.aspx) constructor overload that accepts a Stream.

I think, that's the way to go and the Attachment may look like:
```f#
type Attachment = {
    Content : string
    MimeType : string
    Name : string }  
```