Handled EmailReference

This pull request handles the EmailReference pointer message which has been described in #4.
I think, to be compatible with other clients (e.g. Node.js) we probably have to define the type of the Content as string with a form of [binary-to-text encoding](http://en.wikipedia.org/wiki/Binary-to-text_encoding#Encoding_standards).

For images or audio files, there may be drawbacks with this form of representation though...