Introduced EmailReference

This pull request introduces the `EmailReference` pointer message which has been described in #4.
I use a partial active pattern here because otherwise I had two options that I didn't like that much:

* Rename the `toEmailMessage` function to something like `parseEmailData`
  * Then add another function named `parseEmailReference`
* Parse both EmailData and EmailReference messages in `toEmailMessage` function

Any comments would be great here.