We later want several types of messages: simple 1-to-1-messages, group
messages, and broadcast messages. Message currently had a field 'to'
that was able to hold a single DtnHost. As broadcast messages do not
have any DtnHost, this field should not exist.
Therefore, I split Message into Message and OneToOneMessage,
introduced some abstract messages and added some casts into existing
logic that only works with OneToOneMessages.

refs #40