Made Network.h's Start() and Update pure virtual and removed cpp.

Removed unnecessary code from NetworkDefinitions.h.
Renamed sizeOfPackage to sizeOfPacket in Packet.cpp.

include/Engine/Network/Network.h

@@ -8,12 +8,9 @@
 class Network
 {
 public:
-    Network();
-    ~Network();
-    virtual void Start(World* m_world, EventBroker *eventBroker);
-    virtual void Update();
-protected:
-
+    virtual ~Network() { };
+    virtual void Start(World* m_world, EventBroker *eventBroker) = 0;
+    virtual void Update() = 0;
 };
 
 #endif