Add callback for reliable messages

[Johnscaban]

Added a callback for when reliable messages arrive. This would allow the developer to execute some code after a message is successfully received. This is similar to what it is proposed in issue #42 .

How to use

Pass a callback to the Send methods of clients and servers. In the case of the server's SendToAll, the callback needs to have a ushort parameter for the ID of the connection to which the message was sent to. This is useful in the case that after a message arrived you want to send another message to the same client.

Please let me know if this is okay or if I should change something.