Handled EmailReference

This pull request handles the EmailReference pointer message which has been described in #4.
I don't think we should deal with archived formats (e.g. gzip) if we can use XML files directly...

The drawback of Base64 binary-to-text encoding scheme is that the encoded data is [about 33% larger than the raw data](http://stackoverflow.com/a/14418846/467754).

If with CDATA and/or Bytes the result is the same as with the original file, then we could probably consider that direction.