Support for propagating authenticated client credentials

Hi Mattias,

I have added two fields to the RawData class in order to support the propagation of an authenticated client's credentials. The client identity is represented by a Principal and (optionally) the X.509 certificate used by the client during DTLS handshake.

I have also added code to the Scandium classes in order to provide these credentials during the DTLS handshake. However, this will be another pull request ...