[Network] created base ListenThread class.
[Network] Fixed thread starting in constructor.
[Network] Implemented NetworkService's constructor.
[Network] Implemented NetworkService.start method.

Take note that the signature changed from start() to
start(int port, String group).
It now also throws UnknownHostException if the provided group is not a valid
class D IP address.
[Network] Implemented NetworkService.removePeer method.
[Network] Implemented NetworkService.sendBroadcast method.
[Network] Removed heartbeat from MessageType.

It doesn't need a MessageType as we don't send any data for heartbeat, so we
don't instanciate any Message object for this purpose.
[Network] Implemented NetworkService.disconnectionBroadcast().
[Network] Added NetworkService.stop() to properly close all network Threads.