Handled EmailReference

This pull request handles the EmailReference pointer message which has been described in #4.
BASE64 encoding is probably fine; let's give @sgryt until Tuesday to protest, and if we haven't heard anything against this plan, let's go with it.

If, at a later date, we want to use CDATA, I think it would be possible to add this as an alternative format to BASE64 encoding. As far as I can tell, that would be a non-breaking change.