add WebSockets with STOMP on top

Adds a STOMP broker to the backend. Implements a standalone feedback with messages and topic channels.

This solves #25.

src/main/java/de/thm/arsnova/websocket/message/Patched.java

+package de.thm.arsnova.websocket.message;
+
+public class Patched extends WebSocketMessage<PatchedPayload> {
+	public Patched(String type) {
+		super(type);
+	}
+}