Handled EmailReference

This pull request handles the EmailReference pointer message which has been described in #4.
I was more thinking about whether there would be any problems writing and parsing XML files of that size, on various platforms (.NET, Javascript). There probably aren't, but since we'll need to be able to send emails with attachments of a 'reasonable' size, it's important that it works :)

At the moment, the choice is now between
- (g)zip archives
- XML files with BASE64-encoded attachments inline

Are there other alternatives? Would it be possible, for example, to have include byte arrays in the XML file using `CDATA` elements? If it is, I guess the size would be smaller than when using BASE64 encoding.

@sgryt Feel free to weigh in on this discussion as well :)