Message carbons allow you to receive the outgoing message of another client of the
same account. In short: messages are synced between the clients.

The handleMessage function of the MessageHandler can now handle both types of messages,
normal, direct messages by a contact and forwarded messages (possibly) written from the same
account.

Notfications and unread message counters are only used, if the message written by somebody
else, so you're not getting annoyed by notifications of your own messages. :)

Closes #116.