Introduced EmailReference

This pull request introduces the `EmailReference` pointer message which has been described in #4.
FTR, the 1st option (with the renamed function) is available in a separate branch [here](https://github.com/moodmosaic/Qaiain/compare/add-email-reference-and-rename-functions#diff-512bf7275e6b4260a867ab06fc853933R36).