Resolve "Super Peer should provide proof that specified client has joined him"

This MR adds a signature to every HelloMessage with joinTime > 0. The signature covers the message attributes sender, recipient, time, and joinTime. This signed message can later be used by the receiving super peer to proof to others, that the client is currently registered at it. Therefore, a malicious super peer cannot announce itself as the relay for non-registered clients. For backward compatibility, the signature is currently not used/enforced by the recipient.

Closes #62 (closed)