Added documentation for eventlists

documentation/eventlist.txt

+Event Lists
+===========
+
+Event lists let the user or the system, create named events and users to 
+register for notification when the events are invoked. This is provided by 
+the eventman interface:
+
+1. Creating an event:
+=====================
+event_manager->create_event("event_a"); 
+
+2. Registering and Deregistering Handlers:
+==========================================
+int h1 = event_manager->register_event("event_a", [&] { handler1(); });
+int h2 = event_manager->register_event("event_a", [&] { handler2(); });
+event_manager->deregister_event("event_a", h1);
+
+3. Invoking an event:
+=====================
+event_manager->invoke_event("event_a");
+Registered callbacks are dispached with no particular order.
+The callbacks are exeuted in the context of the thread who invoked the event.
+