Added first helper methods for checking for correct values.
Added ClientConnectMessage, which is used for connecting to a server.
Refactored the message-encoding. Now it's not possible to forget to call
super.encodeMessage()
Moved ClientConnectMessage to a package that only holds the messages
from clients.
Added an Exception for invalid messages. Javadoc was copied from
RuntimeException.
Wrote tests for AbstractNetworkMessage and fixed the class.